What is meant by the "finished size" when placing an order?

"Finished size" is the size the items will be after being folded. 

For example if you require a sheet of A3 paper folded in half to make an A4 newsletter you would need to order as:

Finished Size - A4 (210x297mm)

Sides - 4

Folding - Half-fold

For an A4 sheet folded letter fold you would need to order:

Finished Size - DL (99x210 mm)

Sides - 6

Folding - C-Fold