I have a small mouth so when I smile it looks disproportionate with the rest of my face what Can I do to have a bigger smile?

Healthy looks good. 
If your smile looks ‘small’ , then there is likely value in seeing a physiologic Dentist to see what’s going on.  
Short answers is, with Orthodintics, or Crowns/Veneers or a combination of both... you could have an ideal smile that is both healthy, functional and could be full and wide like you’re hoping for 

Based on your photo, it appears your teeth are in good alignment, otherwise braces or Invisalign can widen your smile by pushing the back teeth out.  In order to produce a "bigger smile", there are a couple of ways you can achieve this.  You want to address what we call the "buccal corridor" or "negative space".  This is the dark space you see between your teeth and your lips/cheek towards the back when you smile.  An esthetically pleasing smile displays teeth that almost rest on the lips, like Julia Roberts.  To eliminate this space, porcelain veneers or composite bonding can be added onto the outer surfaces of the teeth.  You may also want to consider slightly "lengthening" the edges of your upper front teeth.  Generally the edges of the upper teeth approximate the upper border of the lower lip. But at the same time you want to be careful because teeth have an average length of 10-11 mm, so you don't want to make them TOO long.  It's helpful to have 3 other photos as references - a frontal photo in which your lips are slightly open and relaxed (lips at rest) to see how much upper front tooth edge is displayed (rule of thumb is 2-4 mm), one where you can see the entire tooth and gum line to see how long the teeth are, and a lateral/profile view of your smile.