You can see the amount of logic resources available for various devices here http://www.xilinx.com or
http://www.xilinx.com
But even a 24 bit counter is going to require a minimum of 24 registers plus the decoding logic which will vary in size depending on what type of counter you implement,to get 20 Mhz clk speed you would prolly have to force the logic compiler not to use its default generic design so with all the bits and pieces you require an acex or a flex 10K device from altera is a good starting point to look at.