Originally Posted by frantik

yeah with a long top tube you may need a short stem which could make things "twitchy".. it all depends on the bike and the person though. It also depends on the bars. Some dirt drop bars have less extension in the drops compared to road bars

The main thing as far as size goes, as other have said, is you gotta get the drops up high. A smaller mtb isn't going to work with most stems. I recommend going one 1" up from the "medium" size mtb you might ride