Should I show the max. allowed file size in an upload form?

YES. If you do not, people are going to have to guess what file size is permitted and what isn't.

1. What is the maximum file size to upload in Google form?
2. How do I fix maximum file size exceeded in Google Forms?

What is the maximum file size to upload in Google form?

You have options to: Specify certain file types to restrict uploads. Limit the numbers of files uploaded to 1, 5 or 10. Set a maximum file size 1, 10 or 100MB; 1 or 100GB.

How do I fix maximum file size exceeded in Google Forms?

If you need to increase the limit beyond 1 GB, click Change next to This form can accept up to 1 GB of files. At the bottom of the pop-up window, use the drop-down box to select 1 GB, 10 GB, 100 GB, or 1 TB and click Save.