PostEaseText length
A single Threads post is limited to 500 characters. For longer text, an attached text block well beyond that limit is supported separately from the post itself. Check the current limit in-app before relying on an exact number, since these caps have moved before.
Chaining posts for a longer thought
There is no cap on how many posts can be chained together into a single thread, so an idea that does not fit in 500 characters can be broken into a short sequence of posts instead of compressed to fit one.
Images
A portrait crop around 4:5 (roughly 1080×1350) is a safe default for a single image post, matching how much space it takes up in a mobile feed. Square and landscape crops are also supported.