I ran into this problem with properties of type URL. Long URLs are routine. I solved the issue with this bit of CSS in my MediaWiki:Common.css:

/* Ensures long URLs in the SMW factbox don't ruin page layout */
.smwprops { word-wrap:break-word; }

the smwprops class should probably have that CSS added to it in SMW core. That will probably solve the problem except maybe in unusual cases where the properties themselves contain markup that interferes with that CSS. I'm not sure how to handle that, but for now, my "patch" above should solve the problem in most cases.

Resolved with SMW 1.9, see [1].

[1] https://github.com/SemanticMediaWiki/SemanticMediaWiki/blob/master/includes/Factbox.php#L314