I still don’t know how to do it… more than 100 CP to be big? Than they have count bad my captures.
You know those useless stats pokemon have? Like weight and height? Weight is the one for these. When a Magikarp is big a small and blue ‘xl’ is noted above the weight. It has nothing to do with cp/level.
SO the only thing you can do i catch em all. There is no way of knowing before hand, unless you let some1 else do the catching and checking 
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I’m pretty sure it goes by size of Magicarp caught.
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Thanks ,so it’s size or weight only… I will note when my phone isn’t light as usual…
This might help clear things up.
The Silph Road article about big Magikarp and Tiny Rattata
Below a summary of the article.
A “tiny Rattata” being any that are lighter than 2.41 kg and a “large Magikarp” being any that are heavier than 13.13 kg. The probability for getting a tiny Rattata³ is 11.7% and for large Magikarp it’s 14.6%.
Edit: oeps. Copy paste the wrong link. 
. Fixed now.
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