For optimum performance your packraft should have a snug fit. For comfort you may want a little extra space.
To gauge what size best suits your needs, sit up square against a wall with your legs out straight. Measure from the wall to the bottom of your feet
This measurement will give you a rough idea of what sue you will want but soll vary depending on the use of your buoyancy aid, shoes, boots, use of a seat backrest etc.