Measure the complete route
Measure the available counter depth and width, then the sink and storage location. Include the clearance needed to lift the board. If it will slide into a shelf, leave tolerance instead of matching the opening exactly.
Match the working area to the task
A small prep board uses less counter space and is easier to wash. A larger board provides room for long knives, ingredients, and carving, but occupies more of the kitchen. A groove, handhold, or wide edge profile reduces the uninterrupted working area even when the outside dimensions stay the same.
Check weight before increasing thickness
Finished volume grows with width × depth × thickness. On a large end-grain board, a small increase in thickness can add noticeable weight. Ask for an approximate finished weight and decide whether the board will remain on the counter.
Nominal size is not enough. Request finished dimensions including feet, handles, and any projecting details.
When a custom size helps
A custom size is useful when the board must align with a sink, cooktop, shelf, or specific workstation. Give the maker the physical opening and required clearance, not only the desired board dimensions.