I'm not sure what you are after. Is it that you are wanting the dropdown (it's not a combobox, that's a diffeernt control that is not available from HTML) to open on mouseover rather than click?
There is no default behavior on a mouse over event for a select element, so I'm still not sure what you are after.
Taking a giant leap, if you are saying that when you click on a select with fixed width, that the options are truncated, I don't think there's anything that you can do about that if you've given it a fixed width.