Is it possible to add sold by length for fabric retailers for example into the Items field?

You can use the "sold by weight" feature that already exists. As you may have noticed Loyverse doesn't specify measurement units so you can establish any that you like. 

For example you can create an item with the name "Item (m)", writing "m" to define the measurement so you don't forget; then select the "sold by weight" option. By doing so when making a sale you will simply introduce a quantity, which for you will mean meters. 

I hope this helps you.