You can't control the size of the text itself, but the text will adjust to the size of the button image you specify in your theme (ListItemHighlightSD and ListItemHighlightHD), so if you want more spacing, you can specify a custom image with some transparent pixels above and below.
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)