Beets are not all the same size, but you can weight them in the store (if they are fresh, although they will have the greens on them) and most of the store scales have both ounces and grams listed. If you are buying canned, believe it or not metric (grams are metric) is the official measuring system of the USA and all goods sold commercially will have the metric weight listed. So, just check the side of the can.
A beet is technically a root. So the root of the plant is the beet, and whatever that grows out of it is the actual 'plant' portion.
Tap root
root
.
a root
root
It is a Tuber
They are the same, in most cases. Red beet is the common name for beet root sold as food. Beet root is more generic, and can refer to other members of the beetroot family, such as sugar beets.
potato head.
beet root
Turnip is a root vegetable that looks like beet. Beetroot Sugar Beet Parsnip Swede
A fruit contains seeds. A 'beet' is the root of the beet plant, and is therefore a vegetable, not a fruit.