What depth and size is best for a raised garden ?

The depth depends on what your planning to grow. Root veggies like carrots, beets, potatoes require at least 16". Leafy veggies such as lettuce, kale, arugula etc. Only need 10 to 12" At most! I suggest using burlap sacks to grow potatoes, then you cut the sack open at harvest...no digging or damaging the potatoes! Easy peasy. SEED packs tell you how deep to plant, for best results, follow those guidelines. Happy growing.😊

Youre very welcome...if you want to grow root veg in the same garden at low depth soil...you can build a 1' bottomless box to place atop the soil and add more soil to get your depth....my Grandmother with a green thumb used this method too...the box is 12"x14"H. She just pressed it down firmly and added the extra inches of soil, 4 carrots per box. Potatoes too, but the sack is easiest to harvest. Roll sack down, fill 2" soil, Plant seed potato so tip just barely peeks out on top, add soil as plant grows so only top leaves show , unrolling sack as you go until soil is about 18" deep. ( carrots grow well in 2l pop bottles, just cut of neck, fill with soil and grow 1 per). Good luck.