I've ridden 23s and 25s on both gravel and chip seal. I wouldn't ride a lot of gravel with them, but they're fine on chip seal. The 25s are definitely more comfy. You should be fine with 28s.

As for trainer work, most 28s should be ok as long as there's not too much tread. The more treaded the tire, the noisier it will be on the trainer. I wouldn't use expensive tires in any size on a trainer- they wear the rear tire down very quickly.