I believe in the episode where the Decepticons had tricked humans into believing that the Autobots were the ones who were the villains, there was a scene where Megatron took Lincoln out of his spot and sat there instead so that should give you an idea of how big Megatron was and Optimus Prime too for that matter.
Take care and may God bless.

Theres been multiple episodes featuring human characters to scale with the Autobots & Decepticons throughout the entire G1 run. Spike Witwicky and his father being the most prominent and featured throughout the first two seasons frequently.

Yes there have. It's been decades since I've seen the old Transformers cartoons.
Take care and may God bless.