Protean (NSDL) and UTIITSL list the photo as 3.5×2.5 cm at 300 DPI, which is about 413×295 pixels, and the signature as 2×4.5 cm, about 236×531 pixels, both JPEG under 50 KB. The default preset here exports a 413×531 passport-style crop under 50 KB, which the portals accept; to match the exact 3.5×2.5 cm box, use the Exact size mode on the reduce image size in KB page with 413×295.

Aadhaar itself doesn't take photo uploads (the photo is captured at the enrolment centre), but many Aadhaar-linked forms use the same under-50 KB rule.

Try it on your own photoDrop a photo at the top of the page. Ten seconds later it's sized, sharpened and ready to post.
Compress a photo