width define for the combobox

Can somebody know how a combobox ‘width’ define, so that the large of the screen show and not largest.
With other words, when the data very long is, that they can a horizontale balk setting in the combobox. I write with ModeliXe and PHP for the Linux Webserver.
With textarea can that with attribut ‘wrap=off’.
Help me, please.

Anja