lolbridge

first of all, the member between the two bottom nodes (the "tension member") must have the tensile strength to support the force of the point load on top. I use 1/16" x 1/32" bass wood members, something that I have determined from experimentation to work. This size of wood is kind of hard to find, so I would suggest a 1/16" x 1/16" bass wood or a "medium" 1/8" x 1/16" balsa wood (balsa wood is particularly capricious, there are a multitude of different types of grains and the wood itself is not very consistent from stick to stick, so i would suggest you find a stick of 1/8" x 1/16" balsa wood that weighs about 1.5 - 2.5 grams and feels hard to the touch) for the tension member.
second of all, the compression member is the main part of your bridge. if you need help, ask your sister, but in essence, when pushing down on a thin stick of wood, the stick tends to bend outward and snap in that way. Thus, if you so desire to do calculations, you would need to use the buckling/bending force formula with various constants of balsa wood. However, as stated before, balsa is notoriously inconsistent. I would suggest at this moment to use 1/8" x 1/8" balsa wood for your compression members, as this allows for resistance to both horizontal and vertical bending forces.
third of all, the members connecting the compression and tension members. theoretically, these members have no force whatsoever. In fact, their only purpose is to "split" the compression members into various section, because the stick of wood is less susceptible to bending when a shorter length. Experimentation once again shows that the middle two pieces are required to be stronger than the other pieces. The other pieces may be made out of 1/16" x 1/16" balsa but the middle ones should be made out of something like 1/8" x 1/8" balsa.

in regards to the length and height of your bridge, i have found the height of 10 cm to be an acceptable height to start at. also, the length of the bridge should be slightly over 35 cm, so I would suggest 37 cm, 36 cm at the least.