Would anyone know how to decrease the size of the 'Next' and 'Back' buttons? Thanks!

It is going to be hidden within the CSS but you can override that with some custom CSS.
likely one of these to play with but i am not sure sure in your environment:
.Skin #Buttons{max-width:200px;margin:0 auto;padding:40px 0;margin-top:-15px;overflow:hidden}
.Skin #Buttons #NextButton,.Skin #Buttons #PreviousButton{-webkit-border-radius:4px;-moz-border-radius:4px;-ms-border-radius:4px;-o-border-radius:4px;border-radius:4px;font-size:10px;background:#319dab;width:100px;font-size:24px;font-weight:400;min-height:50px;padding:10px 20px}
@media (max-width:100px){
.Skin #Buttons #NextButton, .Skin #Buttons #PreviousButton{width:25%}
}