How detailed should an ideal customer profile be for an early SaaS?

One page. If it's longer than that at the early stage, most of it is guessing. Mine covers: the type of company (industry, rough headcount, where they are), the problem that makes them go looking for a tool like ours, who feels that pain day to day vs who signs off on the money, and a short list of who we don't want, because disqualifiers save more time than anything else on the page. Then rewrite it after your first 15-20 paying customers. Look at who actually bought quickly and stuck around, it's usually narrower than the version you started with.