Hello Nigella. Recently I prepared your magnificent Ginger Chilli Bass Fillets & Sweet Potato and Red Lentil Dhal, a perfect recipe for guests in Cape Town, South Africa. When dicing the sweet potatoes, I wasn't sure how large the pieces should be for an effective dhal. In typical fashion I had left them "too big" (around 3-4 cubed centimeters each), leading me to scoop them out and rechop them on the board, trying not to burn my fingers. Fortunately the wine helped numb any pain I would've felt! Is there a standard chopping size? Personally I prefer the dhal quite chunky to complement the fish, but I'm sure if it's smooth it would taste just as good. Is there a preferred chopping "size" given the cooking time of the potato and the lentils?
For domestic cooks there aren't really set sizes for diced foods and we would not suggest that anyone should feel that they have to cut vegetables into exact sizes. Personal taste is important and if you prefer a rougher, chunkier finish then larger size dice are absolutely fine.
However as a rough guideline it is worth mentioning that denser vegetables will cook more slowly in large pieces and more quickly in smaller pieces so if the cooking time is very short then smaller pieces are probably more appropriate. If a measurement guide is important then we would suggest diced vegetables could be 5mm to 10mm (1/4 to 1/2 inch) and roughly diced or roughly cubed could be 10mm to 15mm (1/2 to 3/4 inch).
If you see the term "brunoise" then this does mean you need very finely diced vegetables. Typically brunoise will be cubes of less than 2mm (1/16 inch).