Pokemon should able to spawn at ANY size, but the more extreme the size (either giant or tiny), the more rare, like a normal distribution.
Alpha-size would be comparable to shinies, but there would be no limit AT ALL.
If you win the size lottery and get crazy shit like a kaiju joltik or a hand-held tyranitar or whatever, that's a feature not a bug.

Also store the size as a float instead of a uint8_t just to make absolutely sure that you can get super crazy shit.