I wanted to warn everyone about controlling your original feed size for feedburner. They have a limit of 512 kb and that is quite a bit. However if you have a large amount of content and you let your feed grow unmonitored feedburner will stop updating it when it reaches the limit.
I don't know any special ways of controlling feed size with wikidot but you can do it all sorts of ways, I suppose. I just use a tag for my main site feed. So that tags can be removed from older posts or a new tag can be chosen, etc…
To check the size of your feed you can go here to web sniffer and enter in your wikidot feed url. It will give you a size in kib.
If there is a way of automatically limiting feed size that would be great.