0.5ml or 1ml lip filler to even out small top lip? I still want them to look as natural as possible. (Photo)

.5 ml will give you subtle improvement, but will improve symmetry. Lip augmentations with fillers is a great option to improve the fullness, suppleness and symmetry of the lips. Results can be very natural-looking and will feel very natural. I prefer to use a micro cannula to inject the lips to prevent bruising. Look for a cosmetic dermatologist or plastic surgeon who is an expert in fillers to discuss your desired results and concerns. 
I hope this helps.

Thank you for your question and photo.  The ideal upper lip to lower lip ratio is 1:1.6.  By keeping this in mind, one syringe of Volbella (0.6cc) can provide a nice result.  An in-person consultation with an expert injector can provide more information.

 Hi, I have performed many lip augmentation procedures using dermal fillers or lip implants for over 30 years. From the photos, your upper lip is small compared to your lower lip. Following my beauty principles, the lips should be full with the upper lip approximately 75 % the size of the lower lip for the lips to appear balanced. The lips can be augmented using a dermal filler or a Permalip implant.  IT might take a tad bit more than 0.5 cc in the upper lip and there's no reason to further increase the size of the lower lip which would just perpetuate the lack of proper proportion between the lips.
Dermal fillers and lip implants offer a much more reliable volume for augmentation of the lips than permanent fillers or fat injections which are associated with increased, unwanted, side effects.
Hope this helps.