Door ones are 6x4, which is an unusual size. With some minor modifications, you can get 6x5 in, or if you build some kind of adaptor from MDF, or similar, you might be able to get a 6x9 in.
Rears are 6.25 I think. Some people have got 6.5's in there, and I know a few folk have build mounting rings from MDF to raise the speaker up and get a bigger size in.