Mike is correct

Intuit suggest multiple unit of measure, it is not implemented well, I discourage its use, see this for why.  http://onsale-apparel.com/Rustler/tag/um-good-and-bad

If it were me, I would stock the sheet of glass as a qty of one, with the cost of one sheet

When you cut it up then it is up to you

You can expense to cogs that whole sheet if there is not much in the way of usable pieces left over, use inventory adjust for this, adjusting account would be cogs

you can wait and monthly do a count of full sheets, and expense what is missing to cogs (my preference)

for sales use  a service item instead of trying to micro manage sheet inventory items in a system not designed for it.