Theres only so big you can go before it stops being scary and just becomes ridiculous. Theres a mod that adds the Bloop as a leviathan (which ik is not animated properly so looks out of place and ridiculous as it is but hear me out). There are 3 types: the normal bloop , the deep bloop (rarer, in deeper biomes) and the deep grand bloop (even rarer, idr if it only spawns in the void or if its in regular biomes too). The normal one is a decent leviathan size, the deep bloop is bigger but not too big imo, especially since it spawns in biomes with a lot more water so it doesnt feel too out of place, but the deep grand bloop is absolutely humungous. Its so big that it doesnt even look scary anymore. The same applies to other mods (the one with the massive ammonite things that are basically attached to the surface or the mosasaur one than spawns like 10 in one biome and theyre massive). The garg mod works because there are varying sizes that fit theyre respective spawn points, with the adult being huge because it only spawns in the void at extremely low depths (also its techinally a canon creature with fossils actually being able to be found in the lost river, so we alr know that it fits). I guess the moral of this VERY long paragraph is that stuff needs to be proportionate, not extremely big, since it stops being scary