find out the size of open terminal

If you issue the command

stty size

it returns the size of the current terminal in rows and columns. Example:

$ stty size
24 80

You can read the rows and columns into variables like this (thanks to Janis' comment):

$ read myrows mycols < <(stty size)

Obtaining the size in pixels requires knowledge of your screen's resolution and I don't think stty has direct access to such information.