That sounds like overkill to me but what do I know... I had some slides scanned recently and they came out to be around 55mb. I was pretty happy with them considering they only cost me $2/scan. You can easily get a poster sized print out of a 6mp digital camera and those files are now where near that size.

I would double check with them to make sure that is the size the need, and if so, ask them if they can recomend a place to get the slide scanned.