Not having used Outlook Express in ages (and, even then, I didn't use stationery), I can't give a definite answer. I don't think it's your resolution, unless you're running at a really low setting. Nowadays, anything northwards of 1024x768 is fine for smaller monitors (below 17-inches), and 1280x1024 for 17-inches and above. (If you have an LCD panel, it's best to run it at its exact native resolution.)
Beyond that, there's not quite an easy solution. Open up the following directory: C:\Program Files\Common Files\Microsoft Shared\Stationery. That's where, by default, Outlook Express keeps the files it needs for stationery. Each stationery is just an HTML file, so you could, in theory, muck around with each one until it works properly.
In general. though, I am not a fan of e-mail stationery, so unless you want it that badly, I would advice against using it.