If you looking at doing the front speakers you can get 17cms in there with just the slightest bit of trimming of the trim. And its definitely worth it.
I have Kenwood PSR 71s components installed, sound is brilliant, retail at £180!!! bass junkies have them at 110. Can never get their online ordering to work so best bet just phone them, if you dont fancy taking the dash off yourself then any good ice place will do it for about an hour and a halfs labour £60 ish. Its worth it they give a lovely sound, Especially with some nice big speakers in the back.
As for the back speakers go, I was advised not to even bother and I just went for a stealth shelf like your fiesta. I put some Pioneer TSa 2510s in there, 10" speakers 3 way speakers!!! They sound great without the in the face bass o 6 x 9s depends what your eventual plan is. If you getting subs then 6x9s detract from them. If subs and amps not your plan then 6x9s or 7x10s on a stealth shelf your best plan.
Good luck