You should choose a hosting plan with at least 1 GB of space or more, depending on your requirements. For my ecommerce site with less than 100 products, I've been utilizing OBHost's hosting package with 1GB disc space.
Not just disc space, but CPU and bandwidth should be taken into consideration. I'd probably start with no less than 10 gb for both (disc space and bandwidth). You should ask yourself, what if my ecommerce site takes off? Then your 10 gb won't be enough. But it'll be a good start. Make sure you install one of their anti-bot plugins too. Bots can drain your CPU and cause your site to be slow or go off line altogether.
You need to consider both the number of products AND the number of sales - as well as the number of bot registrations.