Students cannot alter form sizes within Moodle - your instructor will have to do that. Please contact your instructor about the problem, and your professor should respond.
In most course's the size limit for each activity is set to 64 MB. 64 MB file can contain 1200 pages of text, 40 photos, and 4 MP3 music files - more than enough to submit a homework assignment.
If you need to reduce your file size, please refer to this guide here