Lip implant sizing before surgery can be a challenge because there is no great way to determine what end result will look like before the procedure. It is true that very few patients choose the smallest implants because the end result is typically very subtle and most patients desire more of an enhancement. Sometimes it helps to estimate the size based on the amount of filler used with previous lip injections. Based on your picture it appears that your lips are really not that thin and a medium sized implant would work well. Also realize that if you were not satisfied after the procedure, it is relatively simple to change it out for a larger size.
While everyone's lips are different, most patients who want a noticeable change choose the 4mm Permalip implants. More subtle changes are done with 3mm implants. It is uncommon to need a 5mm diameter lip implant unless already has an indwelling implant.
Thank you for your question and photo! I advise you to have a consultation with a board certified plastic surgeon. They will be able to guide you on what will fit you best.
Dr Dhaval Patel
Double Board Certified