Breast shield sizes

So my nipple isn't that big, I got a mandela swing breast pump with accessories for 80$ off a lady who wasn't able to use it (really good deal, she included bags, pads, soothing patches etc) anyways it came with a standard shield... but looking at it... the tunnel part is the almost size of my areolas maybe 1-2 mm bigger, I'd say the size of a quarter and my nipple is the size of a pea, what's the rule for sizing though? My nipple is like 1 cm (10mm diameter), and the smallest shield on the website says 21mm??? So would that work for me? Or no?