Most people make beds 4 x 8 or 4 x 4 feet. 4 x 4 means you can reach from all sides fairly equally. But I’ve seriously thought about making 2 x 8 beds! The math works out like this for lumber:
A 4 x 8′ bed takes three 8′ boards.
A 4 x 4′ bed takes two 8′ boards but only gives half the planting space of a 4 x 8' bed, so it would take four boards to equal the same planting space.
A 2 x 8′ bed takes two and one-half 8′ boards and gives the same space as a 4 x 4 bed.
I have short arms to go with my short stature, and I can’t reach the middle of a four foot wide bed without getting down on my hands and knees. The whole idea of raised beds is to reduce the bad kind of bending that reaching something at that angle requires. Seniors and those with handicaps have found that raising the beds even higher off the ground means they can continue to garden when they wouldn’t have otherwise
Starting out smart might mean more years of enjoyment before you begin to cope with a rebellious body. When it comes down to it, the boards aren’t the biggest expense and the soil inside will cost the same, regardless of how you shape it, if the goal is to have a certain amount of garden space.
From personal experience I will say that better quality boards will make up for the extra expense. Our one original food pantry cedar bed has outlived two successions of beds made with lesser quality, and shows no sign of decay yet.
This is of course assuming that the walls will be made of boards. There are other alternatives.