JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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In the form builder, click the upload fields you want to increase attachments size for, then click 'Max File Size' button and set the desired limit. The value is in Kilobyte (1024KB = 1MB).
The screenshot below illustrates how you can do it.
Please let us know if you need more assistance!