The thighs are made up of the quardiceps and the hamstring; you need to work these muscles. Leg extensions, leg curls, and even squats can help to target this area.
The best way to increase the size of your thighs is to make your leg muscles larger. The beset way to do that is to walk more. Walking long distances regularly will increase the size of your thigh muscles.
You can build muscle and increase thigh size by doing exercises that target the thighs, such as leg lifts, leg presses, and weight lifting using the thigh muscles. A gym should have the equipment you need.
In order to get bigger thighs you need to work out, running, walking, working out the thigh muscles on eliptical and weight lifting of the legs. It will take time to get the right weight there.