Large print and low resolution will lead to a pretty low quality print.
If you have a file that is 30k x 30k and you are trying to print it 4 ft x 4 ft for example, this will give you roughly 625 dpi for your printing. This should be doable for any print shop. If you have less dots than that you will see the resizing artifacts in the print (pixel blocks).
It should be trivial to open the file, resize it, and then edit it at will if needed. If a print shop doesn't know how to do this I would worry about their ability to print large format imagery.
The good news here is that you likely have too much detail in your map and you can fly a bit higher. The coverage and overlap will be the same but the final product will be of lower resolution.