What do you mean by portable? A version that could fit on a large ship? house? big truck? car? cell phone?
When it comes to tokamaks bigger is generally better for efficiency. I don't see any reason you couldn't build one on a large ship, but I don't think you'll see a portable fusion generator anytime soon.
As far as I know, you need a large volume to achieve a good volume/surface area ratio. If you could increase the plasma density this would become less of a problem and you could make them smaller.
Secondly, most reactions will produce high energy neutrons which must be shielded. The same thickness of shielding would be required regardless of the size of the tokamak.This would introduce a practical limit on how small a reactor could become even if you managed to get very high plasma densities. You also need a lot of advanced equipment to go with it, microwave pre-heaters, vacuum pumps, fueling systems, magnetic systems, control systems and then the turbine/generator.