A 15 will be enough power to fly a 100 inch model with a reasonable payload of perhaps an extra 1.5 lbs over the weight of the glider. Engine duration will be dependent on the fuel tank size. However a .15 will have trouble running consistently with more than 4 oz of fuel. Certainly a 6 oz tank would be asking for trouble. If you need more than the 20'ish to 25'ish minutes of run time the .15 with 4oz will provide then you need to get yourself working with a larger supply tank that feeds a small "float bowl" tank up by the engine. It would literally have to have a float in it and a needle valve similar to the float bowl in the old car, motorcycle and lawmmower engines of old. The idea is that the light suction power of the .15 to .25 needs to operate with a consistent fuel level so it can suck the same amount of fuel for the whole duration of the supply. There is just no way to provide that level of consistency with more than a 4oz tank. In fact in my experience even the 4oz tank is pushing it and you'd need to set it a hair on the rich side while full.

Takeoff is no problem. A handlaunch requires no distance at all. Landings would benifit from spoilers or flaps but a typical landing "slide" for such a model would easily be less than 30 feet.

You may also want to look at something more like a stick built old timer. They come out very light, can easily carry your payloads and can easily take off in 30 to 35 feet even in calm weather and land within 50 to 60. Less if you were to use a custom wheel brake system. It would not have to BE an old timer. It just needs to be built LIKE one so a basic box and rectangular wings is fine. Just don't use much wood.