The size of the backup is determined by the files and database data that you put in it.
Akeeba Backup automatically excludes some files and database data it knows you won't need. On the files front it automatically exclude Joomla's temporary, log and cache folders; your server's logs folder (if it's inside your public root); PHP error logs; PHP core dumps; and Operating System-created folder thumbnail files.
On the database front it will automatically exclude the Joomla sessions table and mySite.guru's file cache which tend to be pretty big.
From that point on it's up to you.
Do you have multiple sites under your public site root? For example sites in subdirectories or even subdomains whose root is a subdirectory of your main site. This cannot be detected automatically and are included by default. If that's the case, exclude them.
Do you have any massive archives (e.g. from manual "backups") that needn't be backed up? Look in your backup log file for "big file" – it will give you an idea of which files are rather big and may warrant exclusion from the backup or even deletion.
Regarding your tables, you may have information you don't want to backup such as Admin Tools security exceptions log (#__admintools_log) or the PHP File Change Scanner's results (#__admintools_filescache, #__admintools_scans, #__admintools_scanalerts). Nothing else in our extensions can be safely excluded.
If you have multiple sites sharing the same database you may want to exclude their tables. Our documentation tells you there are two ways. Either using the Excluded non-core tables in the database tables exclusion page or using the documented regular expression for the same effect (which is better and easier maintainable, in the sense you activate once and never bother with it again).
Third party extensions may keep a cache in the database or the filesystem, or they may have other data you don't necessarily want to keep. Consult their documentation and developers.
From that point onwards your question devolves quickly into the "how long is a piece of string" territory and cannot be replied within any degree of accuracy. All I can say is that if there is any file or database data you are absolutely sure can be skipped from a backup (as in: it won't cause your site to go boom or miss any important stuff when restoring on a new server) then you can of course exclude it with the tools provided in Akeeba Backup.
Nicholas K. Dionysopoulos
Lead Developer and Director