Pective: Compare actual size images of products
One of the big problems with buying online is that the size of products can be wildly mis-represented. There's been a few times I've ordered something and then been surprised at the size of it when it arrives - and no this has nothing to do with Andy's earlier post.
Pective aims to solve this problem by serving up accurately sized images of products. You go to the site, enter your screen size and then it gives you a correctly sized image. Now there are two main problems I had with this. First, I found it didn't really help me get a spatial grasp of the product but I chalk that down to my poor brain abilities. Second, they displayed the image in an impossible small window where I had to scroll every which way to see all of the product. This made it pretty close to totally useless.
So in review - you'd probably be better visiting the manufacturers site, getting the specs and building a papercraft model at home with ruler and pen.