Broke a leaf spring. Shop says I should instal bigger ones?

Let me guess: It broke near the eye of the leaf spring, right? That's where they always break.

I have experience with this as nothing will stop you in your tracks like a broken leaf spring. Most leaf springs used on the lower and mid level trailers are $29 China junk.

Do yourself a big favor and get a custom set of springs made. Replace all 4! They should run about $80-$100 each but if made correctly, they will last forever.