Context menus set their own size, which is based on the size of the skin's font. use env->getSkin()->setFont to set a different sized font.
Thinking about this, perhaps we should have the option for several fonts rather than just one for the entire skin.
Oh ok, thank you, actually i thought we could directly change the size of the font.
So thank you.
EDIT => i have also an other problem => when i resize the window to maximise it the context menu's width is not the same that the one of the widow, so is there a way to solve this problem?
You need to get the resolution of the window after resizing it and then alter the size of your box. I'm not sure of the exact code. I don't think Irrlicht actually has a getResolution function...
this is/was a bug, it is now fixed in svn as of revision 652.
If you want to fix it yourself, change RelativeRect to DesiredRect in the following files/functions-
CGUIEnvironment.cpp : CGUIEnvironment::drawAll()
CGUIMenu.cpp : CGUIMenu::updateAbsolutePosition()
and recompile Irrlicht.. thanks for reporting this