Hello LiddyHeneghan,
Thank you for your question. There is no hard limit for email account size, so you will not be charged more for this. But, be aware that we will not perform daily backups if your total account size exceeds 10GB.
Also with IMAP, if there are a lot of emails in your folders it can cause your email client to take a long time to load.
Here is a link to an article that provides more information regarding this.
If you have any further questions, feel free to post them below.
Thank you,
-John-Paul