Its going to look really really ugly due to filtering and mipmaps and what not, but you can do it by scaling them by the correct amounts.
1. Get the size of one of the sprites
Vector3 sizeA = spriteA.CurrentSprite.GetUntrimmedBounds().size;
2. Get the size of sprite B
Vector3 sizeB = spriteB.CurrentSprite.GetUntrimmedBounds().size;
3. Calculate scale of sprite B to be the same as sprite A.
spriteB.scale = new Vector3( sizeA.x / sizeB.x, sizeA.y / sizeB.y, 1);