2.2.39 [HTML] Section 17.6, The SELECT, OPTGROUP, and OPTION elements
The specification states:
size = number [CN] If a SELECT element is presented as a scrolled list box, this attribute specifies the number of rows in the list that should be visible at the same time. Visual user agents are not required to present a SELECT element as a list box; they may use any other mechanism, such as a drop-down menu.
All Document Modes (All Versions)
When a value of the size attribute is specified, a SELECT element is presented as a scrolling list box. If an invalid value is specified for the size attribute, then a SELECT element with option groups is presented as a scrolled list box with a size of 4. Otherwise, SELECT elements are displayed as a drop-down menu by default.