Writing for the Amazon Kindle

I've decided to write some stories specifically for the Amazon Kindle. The formatting is very easy - just using MS Word, but I'd like to add some basic b&w illustrations for flavour. By the nature of the medium, the stories will be for the 10+ age range. I already have 7 stories in progress - but am trying to work out how many pages would be right. Probably 100 or so for the first few stories.