As a guideline you want to aim for 4 sqft of coop space per bird ideally, so in your case of 25 birds that means a 100 sqft coop or about 10x10 or 8x12...
You can shave that number to 2-3 sqft per bird in some instances but it can cause crowding, health and behavioral issues... Your 6x6 coop simply is not going to cut it in the end as that is less than 1sqft per bird... You also need about 1 linear foot of roost perch per bird, so you have to factor that in, as well, 25 linear feet of perch spaced 12" apart and 12" off any walls... In a 6x6 coop you could squeeze in about 18 feet of perch (without stacking on top of each other resulting in the lower ones being pooped on all night) still short of your needs for 25 birds...
Run size should be 10 sqft per bird, or 250 sqft in your instance... You can reduce it to a point but again you run the risk (very high) of behavioral and health issues if you go much smaller...