you connect the resistor from the wiper (the middle leg) to ground.
That way, when the pot is turned all the way up, you have a load of the parallel resistence (pot and resistor).
5meg pots are avvailable in normal sizes.
You can just as well check out other combinations. The larger the pot in relation to the resistor, the smoother, softer your signal will come in when turning the pot clockwise.
For anything larger (like a 1meg pot) you should use a log pot since if i remember right you get linear pots only up to 5M.
Get some alligator clips and a bunch of pots and try different combinations till you are satisfied. Give unused pots to some diy guy you know for future use...