Need Board Sizing, Fiberglass, and Epoxy/Resin help

I’m planning on making a board for my build using fiberglass on foam. I was wondering if anyone could help me understand how board sizing works. How do I decide how long, wide, and deep I should make the board? Also, what kind of epoxy/resin and fiberglass should I use?

Also, what should I use to mount the foil? I was thinking about using aluminum tubes in the board like MaxMaker did, but I’m kind of interested in if it would be possible to mount a rail system that other boards have. Most builds I’ve seen so far only offer one spot to mount the foil so there’s no room for adjustment forwards or backwards. Is foil placement an issue in the first place?

Board size is pretty much a question of taste and how much buoyancy you want - Kids typically need more, because they get on the board, before they can use the throttle. On the other hand around 1,10 is about the minimum you need to get enough pressure on the wing.
Personally I think ~1,50m long, 60cm wide is about the sweet spot. Thickness depends on the battery pack you intend to use: You will need >1cm below the battery and ~1,5-2 cm for the lid: I would recommend to go for at least 12cm thickness to have some spare room. I have rail mounts on all my boards - just used the inserts from Aliexpress for around ~30€ and reinforced with fiberglass.
But please consider that placement of the mast is very much limited by the power cables and water-cooling tubes - so I guess, that you do not lose much when using aluminium tubes.
Epoxy: Most will do, but do not use the cheap stuff from ebay, but proper marine epoxy.
Fiberglass: depends on the stiffness of your core; I would say you need minimum ~700g/m2 to have sufficient stiffness - but opinions vary.

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Thank you so much, your reply has been super helpful in finding the right materials to start building the board.

I went on the Lift website & copied the dimensions (length&width) of one of their boards. The thickness was driven by how slim I could make the battery case. I used aluminum square tube (like Max Maker) but I hammered solid aluminum cubes down the inside of the tubes to the points where the bolts would come through from the mast. Then I drilled/tapped threads into the solid aluminum. Used TotalBoat epoxy for the fiberglass&gel coat. The epoxy said “not to be used for gel coat) and apparently I should have heeded that advice b/c the gel coat turned an amber color & is prone to cracking. So don’t make that mistake!

Thanks for the heads up. I definately like the idea of using aluminum to reinforce the bolt connections.