My first suggestion would be to start eyballing your poly plane i'm assumimg) when you can estimate roughly how many pixels across this plane is, you can then scale your image in photoshop to the deisred level and then project - but you will probably lose detail, or you scale your poly up to fit the image, - that really is going to be the key to ensure a photo or texture isn't going to be so big or so small that you get a lot of stretching.
Another way could be to make the poly plane bigger and use alpha masks on your photo to mask out any unwanted poly's sticking out. My suggestions have all been assuming you work in and out of photoshop and keep your files as targa files. What i think sounds like you would also ask a few more questions regarding the photo ie the res. can you find a smaller size picture of the sme thing but keep its resolution - that kind of thing. either way you look at it your photo sounds to big and if you apply it straight to the plane and still looks too big and distorted then try the above, good luck.