I wanna share some of my design and build rules of thumb that are just pure experience and there is no aerodynamic or structural calculation nor analysis done for them just my experience which i know they are not perfect at all! yet have worked for me through years.
as far as material I always use 3mm thick birch plywood for my air frame it is really good for laser cutting and cuts precise an clean it has a density of around 670kg/m3 somehow heavy but it has really good strength/weight ratio. all the fuselage bulkheads and outer (skin) panels plus any stringer and longerons and wing ribs and spars are from 3mm birch ply. it applys to all my designs from a 1600mm under 4kg trainer to my large 3600mm scale builds weighing around 15kg.
the ribs are always hollowed out at a 4.5mm thick rim and most of the bulkheads and fuselage panels are hollowed at 12-10 mm thick outer borders and 8-6.5mm diagonal truss links. (numbers depend on size of the plane)
as far spars I always use a box design with 3mm ply on top and bottom (10-25 mm wide depending on size) and 1.5mm balsa as webbing on both sides glues to the outer faces of course some small stringer type 4-5mm spars might run the wing lengthwise here and there just as tension bearing links and supporters of the wing skin sheeting which is always 1.5mm balsa and 2mm balsa for +3mm wingspans . for larger planes there might be 2 webbed spars on at 30% and one on the 60-70% of the wing section . i use carbon tubes as wing joiners and have always used bigger ones in diameter as far as it can fit in the wing :D an over design is better than a failed design
for my firewall I always use 9mm (3 of the 3mm laminated) thick for gas engines form 20cc up to 60cc . for smaller nitro and electric ones 6mm thick would be enough.
for the empennage i mainly use balsa wood and have learned to sort my balsa woods by weight so i can manage the weight balance of the plane and not to go to tail heavy .generally so little plywood is used for the tail surfaces and more strength is obtained by more sheeting.
so far this is all of it i hope it could be help full for beginners and i'm really eager to learn more from pros
keep building and flying safe and have fun !