Lua Code:
self.text:SetText("Height: "..floor(ChatFrame1:GetHeight()))
http://wowprogramming.com/docs/api_categories#lua
On an unrelated note, all of the numbers behind the separator you're seeing are not due to accuracy, but due to the inaccuracy of how numbers are stored on a computer - at least non-integer numbers, but in lua, integers and non-integers are treated in the same way.