Each paid plan takes episodes up to a set length:
Week: 60 minutes
Creator: 2 hours
Pro: 3 hours
The full message names your plan, for example "Episodes can be up to 60 minutes on Week. A bigger plan takes longer ones." In the Upload an episode window it shows as "This episode is [length]. Your plan takes episodes up to [minutes] minutes." The window's subtitle also shows your plan's limit.
You may also see "Episode files can be up to 2 GB. Export the audio as MP3 for a much smaller file."
What to do
Move up to a plan that takes longer episodes: tap the Credits box at the bottom of the sidebar, or Plan & credits in the sidebar. On a phone, tap the menu button (≡) at the top right first.
Or upload a shorter cut of the episode.
For large files, export the audio as MP3.