There's [a bug](https://github.com/wez/wezterm/issues/4708) in WezTerm that it waits for a zero-sized Wayland configure event, so its window never shows up in niri. To work around it, put this window rule in the niri config (included in the default config):
There's [another bug](https://github.com/wez/wezterm/issues/6472) in WezTerm that causes it to choose a wrong size when it's in a tiled state, and prevent resizing it.
Niri puts windows in the tiled state with `prefer-no-csd`.
So if you hit this problem, comment out `prefer-no-csd` in the niri config and restart WezTerm.