You are limited by file size. To use the methods described below, the limit is (I believe) 100 MB. You should be able to post any of those little compressed video clips that your digital still camera takes with no problem. If you have regular video footage from a digital camcorder or a bunch of photos made into a slideshow, you may have to compress.
The only method I am familiar with to compress video is Apple's imovie program. That is what I use to make photo slideshows and to edit videos (there is a PC program called Moviemaker that, I expect, does the same thing). The end result is a very large file but it can be compressed. Here is what you do:
Create your movie
Select 'Share', 'Quicktime
Then select 'Web Streaming'
Click 'Save'. Your video should be compressed to a usable size that can be uploaded to your blog.