I want lateral canthoplasty to horizontally lengthen my eyes. Am i limited to that because of the size of my eyeballs? (photos)

From the photo, it does seem that your eyes are slightly more prominent [bulgy] than your sisters, and there are surgical techniques that can help improve that. However a lateral canthoplasty/canthopexy done for cosmetic purposes rarely, if ever, turn out to have a satisfying natural result to objective observers.I would highly recommend AGAINST having a canthopexy/canthoplasty done, however, if you are unhappy with the appearance of your eyes, I would recommend a consultation with an Oculoplastic Surgeon.

Need a photo of you looking straight ahead.
It does appear that you have relatively prominent eyes which is the main difference I see between you and your sister. It is best to get in person consultation/examination with an oculoplastic surgeon.

There are a few reasons why your eyes differ in appearance to your sister's.Your eyes do appear more prominent, but this is due to your upper eyelids sitting higher on the surface of your eyes with more cornea and sclera showing.  This gives the appearance of more prominent and rounder appearing eyes.  Your sister has a narrower face and her eyelids are slightly ptotic or droopy (at least in this photo) with more lid platform showing. This makes the horizontal length or palpebral fissure look relatively longer in relation to the height.  This gives her more of a sleepy eye look.A lateral canthoplasty will not significantly lengthen your horizontal palpebral fissure unless you already have laxity of the tendon, which I do not believe is the case here.  I would caution you against any procedure to change the lateral canthus.  There is significant risk of having a result that you would be unhappy with.

your case her eyes are more prominent which may have a lot to do with a difference in the anatomy of the bone around the eyes and the eye socket itself.  Canthoplasty is rarely effective and young people when it comes to providing satisfactory cosmetic outcomes.  Photographs of you looking straightforward would probably be more helpful.  In short topi tempted to undergo any sort of canthoplasty because I don't believe it would provide you with the outcome your looking for.  At most and orbital decompression may be helpful but this procedure for cosmetic purposes should be approached very carefully.Chase Lay, MDdouble board-certified facial plastic surgeon

Your anatomy is totally different and it is quite unrealistic to think that you can get a look like your sister's.