There's a fancy equation but you can cheat and do what I do.

Go to Foilsim at http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html and load in the size of wing you think you want to use. The measurements are in feet so be prepared to figure out the decimal equivalents. Then select the lift coefficient readout and set the angle of attack to match the readily available lift coefficient of the airfoil you want to use. Typically this is about .7 to .8 for a Clark Y flat bottomed type and about .5 to .6 for a symetrical airfoil. If you chose something with more camber you may be able to set the angle of attack to achieve as much as a Cl of 1. Now switch the readout to lift in pounds. Then start reducing the speed until the lift matches the expected weight of your total model. That will be the lowest practical speed you can use. It'll fly slower but it'll be near the stall.

If you build a larger and lighter model there is no reason a .15 to .25 cannot easily lift an extra pound along with a 2.5 lb model on a 500 to 600 sq inch area wing. Speeds will be low and it won't have a vertical climb but it'll fly nicely. Adding a one pound payload to a 40 size model is no biggie at all as long as your model is not grossly overweight to begin with.