While Cobblemon 1.8+ introduced native size variations for wild Pokemon spawns, the available config options are fairly limited unless you really want to dig into datapacks/etc.
Brecher's Size Variations adds some additional depth to the system:
The mod hooks directly into Cobblemon's native scaleModifier system, so the built-in summary size category (XS through XL) automatically aligns to your custom ranges.
Automatically converts legacy item from the Cobblemon Size Variations mod by Cudzer to this one
The source code is All Rights Reserved, but you are free to include this mod in modpacks and on servers so long as attribution is given in the case of commercialized modpacks, servers, communities and/or equivalents.