For rims, you need to be concerned with lug pattern, size (diameter and width) and offset.
Offset is where the rim's hub lines up with its center line. I think most 17"x7" rims with offset @42 will work, but don't take my word for it. I have one set of 17" rims with a 45mm offset and it does not rub. I have another set of 17" rims with a 40mm offset and it does not rub. I am no expert on the matter and only kinda read up on it when purchasing the rims, but I think the lower the offset, the more pushed out the rim is and the higher the offset, the more sucked in it is. Like my 17"(diameter)x7"(width) et45 set...where the wheels attach to the hub, has the tire pulled more in towards the suspension, while my set that were et40 was pushing the tires out towards the wheel wells edges. This is important because you don't want this number to be off to the point that you rub either on your suspension or your wheel wells.
Rim size is just diameter and width. Easy enough. I don't think you want to go over 7" for width. I have seen people do 8" on the Fiesta, but that is stretching the tire in order to keep it from rubbing. Just keep it simple and don't get a rim wider then 7". Possibly 7.5" will be fine too, depending on the offset. Seen 18" diameter rims fitted and they are fine, but they just look kinda too big for the car to me.
Lug pattern is 4x108 for the Fiesta. 4 is referring to the number of lugnuts/holes and 108 is referring to the spacing/pattern. It has to be 4x108 otherwise it will not line up with the lug patern on the car's hubs.