A 100 KB upload limit is the standard on job-application forms, profile-photo fields, CMS thumbnails and many web-form attachments.
At 100 KB a photo stays sharp at roughly 1000 pixels wide. If your image is a 12-megapixel phone photo, expect the dimensions to drop a lot while it still looks good on screen.
Forms use many different caps. The related pages cover the neighbouring sizes, and the FileHost.net Image Compressor gives manual control of quality and width.