The company suddenly tells me not to leave starting tomorrow. Is it okay to do that because it's a small company?

A 'dismissal notice' is one of the things you must follow, regardless of company size. Even in a company with fewer than five employees, suddenly telling someone to quit without prior notice can be problematic. However, the compensation and procedures for being fired without notice vary depending on the situation, so we cannot make definitive conclusions here. Write down the date you received the termination notice and the conversation details. Tell 1350 about your situation and ask about it.