The original Printrbot printers (the printed ones, not the lasercut) were designed with the intent of being expandable - but not on the fly. It requires replacing a number of parts. The Printrbot+ is the Printrbot scaled up to basically 2x, I think.
But as mentioned above, the bigger you go, the stronger your requirements for rigidity and precision become.
There are quite a few printers that have expandable designs, and there are huge models (e.g. Replicator XL, the huge Ultimaker) but they aren’t dynamically resizable. The closest are the t-slot based printers, where you could swap rods and belts, but that is a non-trivial operation.
A design that might be really scalable (but slow) would be something like a delta-bot based on cables instead of rods, with take-up reels on the extruder platform, Anchor the cables to three tall things, run the platform around to calibrate it, and print (slowly) any size anywhere you like.
I think it would be possible to make it dynamically resizable in one axis only For instance: parallel rails for the X or y dim (it would not be self expandable, but you could attach to the rails).If you could make a wickedly solid base maybe you could make a delta with swappable pillars.
But practically I don’t think it is doable.
hmmm… imagine a quadrochopter based printer 
a swarm of them each with its reserve of powder material and UV laser or liquid adhesive… expandable and multi head printing also becomes concurrent multi chopter printing!