I've never modified the chat frame myself, but this is what I have come up with in research.
For the size of the window, I believe you have to use SetChatFrameSavedDimensions(frameid, width, height) to set the size of the window.
Since it asks for a frameid it should only modify the size of the frame you tell it to.
If you are looking to set the size of the frame based on number of lines to be shown, you would most likely have to do the height setting by multiplying the number of lines by the height of the current font setting in px since different font sizes have different height values.
you can get the current font size for a window by selecting the 2nd output of GetChatWindowInfo(frameid)
Resources used:
http://wow.gamepedia.com/Global_functions
http://wowprogramming.com/docs/api
https://www.townlong-yak.com/