Thanks, that's great! So easy thing, and I didn' found it
Maybe Google should put this ctrl-enter tip here, next to the windows tip:
http://sketchup.google.com/support/bin/answer.py?answer=95012

Thank you, but I prefer the other solution. For example, I have 40 interior doors in a building. Only two types: 90/210 and 75/210. In Layout, I have to make 40 tags. In opposite, in Sketchup I have to make only two components with tags: one for the 90/210, and another for the 75/210. And with the solution of Gjenio, the direction of the text will be always OK.
The only problem is, that I don't know, how to make multi-line dimension text on a Mac

Thank you, that is exactly, what I was looking for!
I have only one problem: a use a Mac, and this "\n" trick works only on Windows. Do you have any idea to solve this? Thanks!

@gjenio said:

Thank you all for precious advices. I think I've found the tip to go through the problem.
1 - Draw the middle line of the door
2 - Thake a dimension right on that line
3 - Double click on the text and type "lenght"\n"height" and remember to chose Endpoint "None" in the Linear Dimensions Menu
Ecco fatto!