Emoticons are constrained by the size of the font that they're placed within. So (unless someone here knows better) I'm fairly sure that the answer to your question has to be 'No'.
I've just taken a look at the web-based email service that I use. I see that it's possible to change the font size (although I've never bothered to use it).
Perhaps you could change the font size (to make it larger) for just the part of your email that includes the emoticon? If our email service lets you do that you should, indeed, end up with an emoticon which is larger than it might otherwise be.