This is a rhetorical question that introduces what Jesus wants to talk about it. It can be translated as a statement: "I will tell you what the kingdom of God is like." (See: [[rc://en/ta/man/translate/figs-rquestion]])
This is basically the same as the previous rhetorical question. Jesus used it to introduce what he would talk about. Some languages can use both of these, and some would use only one. (See: [[rc://en/ta/man/translate/figs-rquestion]], [[rc://en/ta/man/translate/figs-parallelism]])
Mustard seeds are very small seeds that grow into large plants. If these are not known, it can be translated with the name of another seed like it or simply as "a small seed."