It probably depends on your own definition of "too big".
Below are some links that go into printing iPhone images. How big do you want to print, and how close are you going to be viewing it? Where will you be displaying it? Do you need it to be really good or just "good enough"?
I personally wouldn't hesitate printing 8x10s from iPhone images that were not cropped.
Id say depending on the phone and how many pixels the phone takes per shoot. when id print a lot of photos off of my phone say at the Walmart print center i would choose the wallet size or a normal 8x10 because i didn't want the image to look too "grainy" but to each of there own on that opinion.
There are some apps that can help with your problem. These apps can help you to crop and adjust your photos before helping you to print them. To find this app, you should visit website top1apk. I think they will have this app for you. If they don't, you can look for them in other sites. It's because i dont think google play and app store offer you this kind of app.