Quick answer for this. Use Hull Trace for this. Unfortunately, there is no wiki page for this. But you can easly to search it.
https://sm.alliedmods.net/new-api/sdktools_trace
Players mins {-16.0, -16.0, 0.0} maxs {16.0, 16.0, 72.0}.
Tip: if your coordinate is a floor - add +1.0 for Z axis (for sure, that player does not stuck at ground)