The Spiders are on 1 inch fender washers. They would be a little small on a 40mm, but certainly doable. Here's some pics.
So what we have here is:
Reaper Bones Giant Spider on a 60mm base(upper Left)
2 GW spiders on 40mm bases(Lower left)
2 Reaper Bones Vermin Spider on 40mm base(far right)
2 D&D spiders on integral bases, about an inch(middle lower)
So you can see, the Vermin Spiders are a little smaller than the GW spiders, but it would still work. In fact it would give you lots of room to model a nice base. And you can't beat the price. In the U.S. it's $2.50 for 2 spiders.
More pics:
Hope that helps