As we all know that Devil Fruit users are divided into 3 : Zoan, Paramecia, and Logia. And I speculate that each Devil Fruit has its “limitation” to avoid being overpowered.
I speculate this emphasized on the canonical Devil Fruit; Bara-Bara no Mi. It was once stated that the unit for Buggy’s power is Bara-Bara; and his limitations is 200 Bara-Bara. So within the radius where his feet stand, Buggy can separate and levitate his limbs as he wishes, but outside that radius, his power will be useless.
Previously I speculate about Robin’s Hana-Hana limitations. For those who hasn’t read my speculation on Robin, I will make it simple here. Robin can bloom her limbs anywhere as many as she wishes on any surface; and even recently she can bloom thousands of limbs and even her entire body. But once Oda stated that Robin could bloom“200 Hana Hana”.
Because it was pre timeskip, fans speculate that Robin could sprout max 200 Hana-Hana, and since now Robin can bloom thousands of limbs & her entire body, the 200 Hana Hana doesn’t apply anymore. Actually it still does apply. That’s the maximum radius around Robin’s body where she can bloom her body.
Robin’s height is 188 cm, so approximately her arm’s length is 100 cm/1 meter. 100 x 200 = 20.000. Robin’s Hana Hana can only be effective within 20.000 centimeters or 20 kilometers. Approximately her power will activate 10 kilometers in ball-shaped radius with her real body as the central. So although Robin knows exactly how Alabasta is, she cannot bloom her limbs on Alabasta (as she is currently heading to Wanokuni) because Alabasta is not within her reach of power to be effective.
So I speculate that Luffy as well has 200 Gomu-Gomu where that is the max length he can stretch his body. Luffy can stretch 200 Gomu Gomu away before he can probably feel pain on his outstretched body. To count exactly the same with Robin, Luffy’s 200 Gomu-Gomu is 10 kilometer long.
This 200 Gomu-Gomu will be a reason for Luffy’s final Gear where he can probably expand his entire body into gigantic shape. Note that this final Gear is different with Luffy’s Gear 3rd where in Gear 3rd Luffy could only expand selected body part. Luffy’s final Gear form is probably Great Ape form. As his namesake, Monkey. D. Luffy, probably Luffy’s awakening / final Gear is the Great Ape. It’s also as the dedication to Akira Toriyama as Oda himself is a fan of Dragon Ball.
So from Buggy, Robin, and Luffy, I speculate that Paramecia type unit is 200, added with their Devil Fruit power. For example : Hancock’s Mero Mero no Mi is 200 Mero Mero. Probably Hancock’s Mero Mero will be effective within the radius where people need to see her beauty for her to petrify the object. Or Brook’s Yomi-Yomi no Mi, where he can separate his soul only within 200 Yomi Yomi before he needs his soul to be back to his body.
The unit limitation also applies to non canonical Devil Fruit. For example from One Piece Film: Gold. Gild Tesoro’s Goru-Goru no Mi is 200 Goru-Goru. His ship is 10 kilometers long. That’s where his power can be activated because it’s within his radius. This also applies to Baccarat’s Raki-Raki no Mi; where she probably can absorb luck from 200 people before it is overlimited and could probably backfired.
Some Paramecia has bigger range of the unit; for example Viola’s Giro-Giro or Fujitora’s unknown Devil Fruit. Viola’s Senrigan is 4000 kilometers away, but it’s possible that it’s within her 200 Giro Giro. So she can’t use her power above 4000 kilometers/200 Giro-Giro. Fujitora’s unknown Devil Fruit which control Gravity is probably 200 as well as he needs a certain distance to summon a meteor to fall impact on land.
Now that’s for the Paramecia class. How about Zoan and Logia type? Logia type is indeed the strongest Devil Fruit as it’s the power of nature itself. But of course the power of nature has its limitation. Probably for Logias, they limit is 300. Smoker’s smoke body can be effective within 300 Moku Moku. Sabo / Ace Mera-Mera can burn anything maximum 300 Mera-Mera. Even Akainu’s Magu-Magu; especially Ryusei Kazan; can only be activated within his radius. Although Akainu launches his Magma fists to the air, it can be assured that his fists will fall around his 300 Magu Magu range.
Another possibility is that for Logia users, instead of range, their unit is for how many nature force they can produce / control within their area. Logia users can control nature force of their own to amplify themselves, but of course they have their own limitations as well. So it’s possible that Logia users 300 unit is the maximum element their body can produce / control. This is emphasized on Punk Hazard where the island is complete opposite from each other; and using the full reach of their limit will permanently leave a whole area to be affected with their power.
The maximum production of Logia users can also make them become vulnerable to attacks, although some elements maybe superior to the other. Kuzan and Sakazuki are vulnerable to each other, although in the end Sakazuki is superior to Kuzan’s Hie Hie no Mi that makes Kuzan lost one of his leg; but Kuzan managed to inflict damage to Sakazuki as well. So it’s possible that both Sakazuki & Kuzan used 300 Magu Magu and 300 Hie Hie in their 10 days fight to affect the whole area and left huge damage on both side.
So for Logia, the limitation is another “weakness” to make Logia user not to be overpowered. So probably the Logia user can control the whole island with their element, but once again the limitation is not the range / radius; but the production of elemental nature the user can produce before they overexert themselves and become vulnerable even with common attacks.
As Zoan is the “weakest” Devil Fruit. So probably Zoan Devil Fruit user will have 100 as their unit. Or it can be possible that Zoan is the only Devil Fruit which has no unit as their limitations.