Is the software Stumble Upon takes too much space in the computer?
It doesn't take up much space. It is an alternative to Internet Explorer that doesn't have all of the security holes, hijack exploits and other problems that IE is known for. Many computer experts recommend its use.
How good is a Stumble Upon recommendation for a blog?
Without knowing whether your traffic was the same as usual or better than usual that day, it's hard to say whether it was useful. If your traffic spiked quite a bit then I'd say it's a hell of a good thing. If it remained the same, then there wasn't much benefit in it.
I have a self-hosted WordPress blog and I monitor my traffic and referrers on a daily basis. Gives me a much better understanding as to where people are finding me.
Does anyone know how I can install Stumble Upon Toolbar on Opera Browser?
Doing a Google search seems to indicate that Stumble Upon does not have a toolbar for Opera. Here is a site that provides stumble upon support for Opera. I haven't tried it, so no guarantees. Use at your own risk: