I have no idea what my bust length lap sleeve size is?

okay im trying to buy this leather jacket on this asain clothing store. it said "free size" but i dont even know wtf that meant but it had a size chart with bust sleeve length and everything.. i didnt know my body measurements and i dont have the right equipment to measure so i asked the seller.... she says idk. im kinda like wtf. well anyways can someone please make like a estimate measurement of 170cm girls? i am 170cm (5'7) i am 120 pounds. pretty skinny . please help. thanks