To me this question was very clear from the start, but I think too many people read the original title too literally and neglected to read and understand the first sentence of the body.
Most of the comments suggest that people voted to close because they thought the author was asking for three numbers to be converted from mm to inches.
The question hung on without any more close votes for a long time after I pointed out the question was asking how to determine an appropriate nominal lumber size based on the desired final dimensions.
Finally, this morning drs attempted to clarify the title, but when the question rose to the top of the front page, it managed to get another close vote.
After a couple more edits from Matt and another by me, I'd like to know if anyone still thinks it is possible to interpret the question as a simple unit conversion.