How do you chage the size of the unit in the editor

[Gunfreak]

When I add an unit I get the "regiment"  button, how do I change it to brigade?

[Count von Csollich]

you can only add your regiment to a brigade afterwards...
 IF you are referring to the "Brigade" name in the units tab in several of the historical battles....I think this happens due to the fact that a corps can only have a limited number of units...and thus some units have to be merged...by renaming a regiment into "brigade"...
I hope this helps 

CvC

[Gunfreak]

I know that, but when I was making a 300CEH battle I only got 100 regiments, then I couldn't get any more unit, but I had only 160 CEH, the means I was only half way to the total size of the battle,

There has to be some way to make brigades not regiments so you can get the full 300 CEH

I had only about 120 000 soldiers and 250 guns, thats far away from the max of about 250 000.

[Ras]

If you have reached the unit limit the only way to increase the CEH is to add more men to a unit already in the OoB.
This is done by using the sliders on the unit card on the right of the screen. You can adjust the number of battalions, which increases the number of men automatically. Then you can fine tune it with the men slider. You can also add regimental artillery that way (not for all regiments)