<select> does not size by "border-box" model. When assigned height/width to a select element. It was treated as "content-box". e.g. <select style="height:30px;width:40px"> The whole width will be 40 + 1*2 = 42; The number 1 is the default border value.
Loading up a page with a <select> styled as given, the element is 40px wide now. We must have fixed the error sometime since the bug was reported. I'm closing this bug, but if the problem is still around, please reopen the bug with a new testcase illustrating the problem.