Is it teaspoons full or teaspoonfuls?

Both teaspoons full and teaspoonfuls are acceptable but teaspoonfuls is less commonly used. Teaspoonfuls is just the plural form of teaspoonful, so it is more precise to use when you want to specify that you are talking about multiple teaspoons. However, teaspoons full is more commonly used in everyday speech, so it may be more familiar to some people.