You could have multiple sprites and swap once a sprite hits a target scale, but that seems unnecessary to me. You should design your sprites at their largest size, and zoom out accordingly. The only reason you'd need to swap out sprites at the smallest size is if you have a ton of detail which gets really distorted and ugly as you scale the sprite down. However, I think that your art should have the requirement of being able to be scaled easily without losing significant visual clarity.
Some technical considerations to keep in mind:
If your collision detection is coupled with your sprite then the scaling shouldn't impact how collision functions. However, if you've decoupled your hitbox than make sure to update the hitbox size as you scale the sprites.
Since your scaling is going to be global, a basic camera class is a must. Remember, as you scale out your characters aren't the only things that will scale. The world will need to scale as well for the effect to look natural. Depending on where your registration is located on the sprite, zooming out will require some readjustment of the sprite's position. Make sure to keep track of a world position, and a screen position to make this easier on you.
I'm sure there are other considerations to take into account but these are the two that I would deem most important for a first pass of the system.