Is there any set criteria for web-hosting plan resources like sql, mails etc.
Well, you can use as much SQL space and e-mail space as you want under your total Summary Disk Space. You can set the criteria for these resources according to your website, if your website is database driven then you must use, good pecentage of the space for it. Let us suppose you have got 500 MB of space, then you must use 100 MB of space for SQL.
For emails, if you download your mails frequently your usage will be next to nill.