Yes, you are allowed to increase the size of the base, just not make it smaller.
This is expressly for your purpose of making your model's base more awesome.
And especially because the Space Wolf terminators come on a "small" base compared with current terminators, no one will bat an eyelash if you switch him to the normal terminator base.
One rule of thumb is one size bigger. It would be a little weird to see a power armored sargeant on a dread base, but you should be fine.
