For beginners, yoga props can help yoga primaries to complete poses that can't be completed in general, and the use of props does not allow the body to work beyond its own ability, so that the body receives minimal pressure.
For higher-level yoga practitioners, yoga props can be used to try more difficult asana. Yoga bricks are often used as extended hands, sometimes to support the head and buttocks.
Yoga bricks are generally about 3"*6"*9" or 4"*6"*9".This should be a personal choice, for example, your hands are small, then consider choosing a slightly smaller yoga brick. On the contrary, choose a larger, of course, flexibility will be limited. Just considering which size suits for you becaue everyone is different.