Also, it's a simple, yet parameterized, physical system: roughly, you just have to count how many same blocks/beams you are from nearest blue block. This is only a first easy insight, as distances between mass centers are actually computed (e.g. 2x1m vs 2m blocks), and angles also play a role; follow the link below for more details. Additional reinforcements won't work if they don't shorten this distance. Moreover, as far as wooden blocks are considered, distance to ground (blue level) is reset as soon as they connect to stone blocks or better (follow link below to understand why). That means that you can build a wooden shack on the very top of a ruined stone tower, etc.

An excellent reference with game data:
https://www.reddit.com/r/valheim/comments/lxh2d7/building_stability_guide/

Visual depiction of max beam spans:
https://www.reddit.com/r/valheim/comments/m6mlvg/comparison_of_different_supports_before/

Don't forget that a wooden roof is required, so actual max height is reduced! For example, using a core wood frame, a 8mx8m tower can have a 4m high basement, two 3m high floors, plus a comfortable attic under the roof.