The indoor temperature in the unheated case will be just the low pass filtered outdoor temperature. In steady state, therefore, the indoor temperature will be the same as the outdoor - at first glance. However, sunlight heating one or two sides or coming in through a window can make a big difference. Basically, there are too many variables to give a reasonable prediction in the real-life case.
However, we can do the math on the parts you have told us. I think those values are highly suspect, since the overall effective insulation will be quite different from just what the walls are rated as, but I'll use your numbers anyway. This is less of a useful answer to your particular problem than to show how to analyze something like this if you did actually have all the pertinent information.
The common "R value" rating is how many °F rise needs to be across one square foot to transfer 1 BTU per hour. You say to approximate the room as a square box that has 25 m² floor and ceiling area, and 2.2 m wall height. The box therefore is 5 x 5 x 2.2 m in dimension. It's not clear whether the floor should count towards the heat loss. Let's ignore it for this analysis to simplify things. Each wall is (5 m)(2.2 m) = 11 m², and you have 4 of them, so 44 m² for the walls. That plus the 25 m² roof is a total area of 69 m² = 743 ft².
Now we have to find the temperature difference that is to be maintained across this surface. You say you want to maintain 25°C = 77°F (wow, that's wastefully warm!) inside, but that's only half the spec. We also need to know how cold it is on the outside of that wall, which you say is 0°C = 32°F worst case. That means the temperature difference is 45°F.
So now we've defined the problem as how much heat power does it take to maintain 45°F across 743 ft² of R-2 insulation. From the description of R-value above, you can see that each square foot will transfer 22.5 BTU/h.
(22.5 BTU / h ft²)(743 ft²) = 16.7 kBTU/hour
So, you want a heater that can put out 15,000-20,000 BTU/hour. This is a spec that should be directly given for any commercially made heater.