Do you mean you did an internal simulation inside a hollow box or that you did an external simulation and defined the simulation region as being 1ft on each side?
An external model will allow the air to escape and spread the heat into the infinite surroundings.
An internal model will keep that air inside the box and increase the temperature more drastically.
Assuming the analysis is an external analysis, 1ftx1ft could be enough if your pcb is small. But if you're concerned that the bcs are affecting the results, just make it bigger.
If you're forcing the condition using an internal analysis, remove the box and switch to external.
If you are using sim pro thermal, get rid of the modeled air and use a convection coefficient.
I'd also suggest looking up external analysis in the swx kb and going through all of tutorials. There are external flow examples that will show you the basics. Combine with the conjugate heat transfer tutorial and you should be good to go.
This all being said, maybe 70deg c is the temp that your model gets to. Don't forget about things like radiation etc.