A 4300\4500 card to a 6850 will knock your socks off
Just make sure you have the power for it. AMD recommends a 500 watt power supply, but a good(not those 25$ 600 watt units that have less power then a 300 watt unit) quality 450 will do just fine if the system(with power to spare) is not overclocked to the moon.
As for size, It will depend on your case. The 4300 cards are in fact quite small. It is about the same size as a 5770 and those fit into many cases.
What computer(more so what case) are you running?