The only cameras that will give you the full 3.25 x 4.25 image size with instant film are the Polaroid pack film cameras, like the Automatic 100 and the 250. Most Polaroid pack film cameras are rangefinders with automatic exposure, no manual controls. You could also use a press camera with a pack film back, like the Mamiya Universal or the Polaroid 600SE, both of which are fully manual rangefinder cameras with excellent quality lenses. More info here:
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Another excellent option is to find one of the older manual Polaroids (like the 110a) that has been modified to use pack film. This gives you the full image size with manual controls and an excellent quality lens and shutter. I have one of these and I LOVE it. Here is one source (not where I got mine):
http://www.polaroidconversions.com/packfilm/
Polaroid (and Fuji) also made a film back for the 3.25 x 4.25 film that would fit into a 4x5 camera, the pa-145.
Fuji Instant FP-100c is the only remaining instant pack film still in production. It's a color, 100 iso instant film. Fuji's excellent black and white film, FP-3000b, was discontinued about a year ago, though there are still packs available. I don't buy outdated Polaroid film because the results are very unpredictable.
There is a group working on bringing back Polaroid's Type55 black and white 4x5 film. It's not yet in production, but they seem to be getting close.
http://www.new55.net/