to be honest it depends on what the font is for. If it's for the fine text at the bottom then small as you want, as long as it's legible. It's all about how you want to typography to work on the poster. If you want the text to stand out then make sure it's a good size font. I would just say play around with it, stick it on 100% on screen and stand from the other side if the room and see what works. The font size has to work with the layout. Sorry for not being too specific but you can't really say what's too small, it's all about how you want it to look and stand out.