For a mix of road and trails like you describe I think you would prefer to be in the 28 - 38 mm range. You could even go wider if the rims are wide enough, for road use it is not the width so much as the tread that becomes a "problem". Knobby treads add more rolling resistance than width does on a road and some find the noise and handling objectionable too but a wide, smooth tire can be quite fast and pleasant to ride. A trail like you describe is fine with 28 mm tires if it is hard packed. A 28 mm tire will handle some fair degree of softness and gravel even for at least short distances, I've not tried long distance riding on such surfaces and 28 mm tires. I have done a loop of about a mile on 28's on a mix of turf, packed dry dirt, moist slightly soft dirt, pine needles, and a fairly steep gravel up grade and the 28's were not an issue for steering, traction, or control. If the surface will get quite soft in places or during some seasons a wider and knobby tire might be called for. Are horses allowed on your trails? The bike path near me is packed limestone on its far north end and the 32 mm tires I currently have are great with it except that it is all cut up by horse hooves. It is literally like riding on cobblestones. I don't ride that section often and not very far yet. It may be that lowering the pressure in my 32's will make it more bearable, but if you will be dealing with a lot of surfaces like that you might want to go up to 38 mm. On the other hand if your packed trails are flat and smooth the 25's will probably do for now and you can just go wider when you wear them out. There is no harm in trying them for a while to see how they work.

Ken