What is a standard bed, and do I have to use one?

It is the one bed size you use across the farm, and it saves you typing and counting: every new field block starts from it, and you plan 11 beds of beets instead of 330 bed metres. Set it once in Settings > Units & beds, under Standard Bed Dimensions.

You don’t have to use one. If your beds vary, plan in length. And a block whose beds differ keeps its own dimensions, set when you create or edit it.

See Create a field block.