Its not absurd at all! Most part places seem to leave out the scale and just tell you the measurement of the bell itself, so its easy to be a little lost.
Depending on how you want to go, I find most "Medium" sizes work good for a MG, "Small" works good for 1/144th and Large are for 1/72 or 1/60.
However, I tend to like my Thrusters exaggerated a bit, so I usually use "Large" for most Gundam Type backpacks, but stick with Medium for GM/Zeon Mass Production Suits.
I've been picking mine up from HLJ - here - http://www.hlj.com/scripts/hljlist?rel=nav&GenreCode=Sci&MacroType=OptKit&Scale=NonScale
Mechaskunk used to be my go-to guy for most metal detail parts, but he's really only selling bulk orders at this point I think.
In terms of Brands, I like G Temple or HI-Q parts.
I hope that helps, send me a PM if you need more help!