Shopify sets a file limit for line item properties of 20MB. Files under 20MB can be uploaded, but files of over 20MB will error out. If you need file uploads larger than this, we recommend getting in touch with a Shopify guru to ask if they can increase this. You can, however, have multiple upload fields added to the same product.
There are no restrictions on file type for the file uploader in Product Customizer, so you can allow whatever files you choose. These files will not count against your Shopify storage limit. Uploaded files get saved to Shopify's CDN, and while they are associated with the shop account, they are not counted towards the shop's total storage limit.
If you want to set a notice for customers on this limit, we recommend including a note in the field description to inform customers.