There are some square/cube laws that put a rough cap on how big an efficient animal can be with Earth biology. But if you change some parameters (lower gravity, energy-rich environment, stronger building materials than bone) you could get hundreds of times bigger than humans. Even on Earth, intelligent dinosaurs would have been completely possible.

There's a natural limit to size due to the square-cube law. An increase in height leads to a cubed increase in mass which means they are pretty quickly too large and require too much energy to sustain themselves.

There's no reason to believe other sentient life will be in any way relatable to us. Most significantly, there's no reason their perception of time is on the same order as ours. We could be glacially slow or impossibly fast for them to engage in any meaningful way.