Thanks for taking the time to write. I am not entirely sure why FileVault makes it impossible for you to access your email. Indeed, should the email be stored on an IMAP server, you should be able to access it from any machine. Should it be on a POP account, transferring it to Hotmail should not require that you disable FileVault.
Nevertheless, the answer to your question on how to disable FileVault here is quite straightforward: since your iMac needs to make an unencrypted copy of your home folder before deleting the FileVault sparse image, it will need as much free space on your hard drive as your Home folder currently occupies. Hence, if your home folder takes 30 or so GB (a wild guess given your entire system uses 53 GB), you will need 30+ GB free on the drive.
It therefore seems you will need to delete files from either within your vault to shrink it or outside to free space on your drive. Then, your iMac will have enough free space to perform its copy operation and, hence, disable FileVault smoothly.
I hope this answers your question,