When I first started turning I ordered a set of small sized tools... at the time it made perfect sense to get the small tools to go along with my small lathe. Well, in time I ordered my first full sized tools (a 1-1/4" skew and a 3/8" bowl gouge) and then I realized that the smaller tools were not a very good investment.
I find that with the larger tools I have less issues vibrations or chatter and the longer handles really improve tool control and you also get more leverage. You also have more options to adjust your grip to cut down on fatigue when spending long sessions at the lathe.
I guess one could argue the point that if you're doing small projects (such as pens) then small tools are fine, but I do plenty of small diameter spindle projects with my full sized tools, and I personally can't see any benefit in using mini or micro tools for turning pens or spindles.
All that said, if you're turning the inside of small boxes or bowls, then sometimes the little tools do come in handy.