I picked up my first GasGas this year, a 2013 XC300E. I'm 6'4" and the bike fits me really well. I put the clamps in the forward position and did bar risers with Fasst Flexx bars. I came off of a KX450, so I'm comparing it with one of the larger bikes out there and I think it's every bit as big. The cockpit ergos are better for me on the GG though. I did a tall soft seat foam but ended up cutting it down about an inch. Honestly I doubt it matters. I don't sit much.
I also rode a 2013 Beta 300rr this weekend and that thing felt tiny.
As far as parts mismatch, I'm not sure what you mean? It's all GasGas and all the parts are available from the dealers.
Overall I really like the bike. My riding has improved dramatically compared to being on the KX.
I haven't owned any of the new KTMs or Huskies to compare it with, but I did have an 04 Husky TC450 and that bike was mostly just terrible.
Anyway, I hope this helps a little.