why dont you buy one box and then a packet of the size up? i know baby love do an in between size which is 3-8kg or something like that (as well as their newborn ones) - i found these great when he just grew out of newborn as these fit but werent swimmimg on him. I knowyou use huggies but i find these excellent ad could be perfect for your situation as they will fit for a while and can be used when they are tiny too.
FWIW I used about 2-3 boxes equiv of newborn nappies with DS2 who was 3.19 kg (so a touch under 7lbs)