UL That Allows Only A Set Limit Of LI Elements?

I want to create a UL that has a certain amount of LI elements, but instead of showing every LI element, I want to either display 5 of them or whatever the containing DIV will allow just before activating the "overflow or scroll".

So in other words, let's say I have a div with a height of about 300px. If the UL generates a few list items, in turn, creating a UL that supersedes this height value, a scroll bar will be generated (depending, of course, on how the CSS and whatnot is written). So, what I would like to do is generate a "Read More" link at the bottom of the UL and display only whatever number of list items it can take that will fit appropriately within the DIV element.

It was my understanding that things like this are usually done with JavaScript because I can't see how PHP would govern the height of an element unless you somehow replaced the height attribute of the CSS with some sort of PHP variable (which would mean I would have to change the CSS extension and do a bunch of stuff I think would be outside my scope at the given moment). What do you think? What's your perspective on this?