Hello @FI11 and thanks for your question. You can create an item for each kind of flooring you sell. For example, if you sell vinyl tile flooring at $2.50 per square foot, create a new item, and for the unit of measure, search/type for "per square foot". Select this option in the unit of measurement.
When you save this item, you can use it in your invoices or point of sale transaction. You would bring up your item (in this example I created a Test Tile item) with the cost of $2.50 per sq foot. You then add the total number of square feet you are selling.
As far as Square calculating how much flooring you would need to order...I do not think Square can do that. You would either have a paper floorplan chart with measurements and manually calculate the length x width to obtain the square footage, or use another commercial program for measuring or planning the square footage of a room. I hope this helps answer your question.
