Which series/mfr's kits are you referring to? The Hasegawa 1/72 and 1/48 Valkyrie kits are just like airplane models, with good detail and hundreds of little stencil decals to apply. Wave also makes Valkyries, but in 1/100 scale and to a simpler standard. They also do a few Destroids (Tomahawk, Defender, and Phalanx) in 1/72 and are just about as complex as the Hasegawa kits. Bandai periodically re-releases older Imai kits, which need more work to look good.
Fit of the newer kits is generally good, but not perfect. When I built Hasegawa's YF-21, I encountered all manner of tiny fit problems, despite the relatively simple parts breakdown. The large number of stencil decals drove me batty. But I'm a guy who is a stickler for fit and has a short attention span. 
Your mileage may vary.
The only size comparison I can comment on is that the VF-1 Valkyrie is about the same as an F-16 (a friend had built 1/72 versions of both and had them side by side in his display cabinet.) I would imagine the YF-21 is around the size of an F-15.
HTH