How can we go about deciding an appropriate filesize upload limit?

Our web application includes a form where a user can upload supporting documentation. It is anticipated that many users will use the camera on their mobile to snap a photo of a receipt or certificate to upload.

The server imposes a per-file file upload size limitation (upload_max_filesize). Beyond grabbing an array of devices and seeing what size image their cameras output, what factors should go into this decision?

(For example, larger file sizes could slow down submission time for the form leading to an unacceptable user experience, smaller ones could lead to frustration if they're not large enough to accommodate the typical mobile phone camera output.)