All you do is just leave it running and it the mail will eventually download to your computer. If you cancel it part way through you could cause the mail file to double in size, which will mean it will take longer to download. Check to see that you don't have call waiting on your line. Check to make sure your don't have another handset or answering machines plugged in on the same line. Check your modem is not set to disconnect if idle after 15 mins.
Extra space is available at a small fee.
You can check on your account usage by going to the relevant page. Type in your username and press check time - this will email a response to you. Check your email in a couple of minutes and all your usage details will be there. A cache temporarily stores the information on a page in your computer. The first time you ask for a page, No pages are permanently stored in a cache.