You will find that fiberglass is very heavy. A structure covered in cloth will be much lighter.
EVERYTHING in design is a trade off. Your wingspan question is a good example. A longer wingspan is desirable from a reduction in induced drag (drag due to lift), but as the wingspan increases (wing chord will decrease to keep the same wing area), wing weight will go up significantly because the spar height is decreasing so the spar caps have to get much thicker. Design is not easy; it is the result of what you determine to be the best compromise.
BTW, if you try to do it all, you will probably end up with a design that is poor at everything.
Get a book/magazine of existing gliders (or look online). Simple gliders like the “Bug” and “Goat” come to mind. See how you can improve on those designs. There are also some really cool foot-launch sailplanes (but you’ll need a cliff (hill) to run off to start. The more high tech the design; the more it will cost, too.
Look online at what are called “secondary gliders”, too. They are primary gliders with a shell (or canoe) for the fuselage.