In an email message, which would be better to say:
How much is the folder size?
What is the folder size?
How much? asks for a quantity, and suggests you're asking the price of the folder. Ordinarily we say
What size is the folder? if we are asking for a 'stock' size (e.g., 'letter' or 'legal'), or
How big is the folder? if we are asking for dimensions.
These are usual with most 'primary' qualities, such as size, color, height, length, weight and the like.
What color is your umbrella?
How tall is Bob?
What length plank or How long a plank do you need?
What weight does he box at? or How heavy is he?