How tight should a seatpost initially fit?

I have an unmarked sorta uban/mountainy frame - unbranded and no seatpost or other measurements stamped - and was going to try to figure out the seatpost diameter somehow. Bored, today I decided to throw on my 27.2 post and clamp from my Felt - voila! it seems to work, so at first I thought, great, its a 27.2. But, now I'm thinking that a post that was one or two sizes too small would work if the clamp held the tube together tight enough. So.... how tight should a seatpost fit into the frame before one puts on the clamp. I mean, this slid all the way down to the seat, but with the clamp on it holds fine even with my weight on it. Also, if 27.2 is too small, what are the consequences of riding on this?

 

On a new bike a properly sized seatpost will often slide down into the frame by gravity alone without the seatpost clamp to hold it.

 

If you can beg, borrow or blag a vernier caliper, you can check the post size acurately, and presumably put your mind at rest. Personally on my (7) bikes, I have found that if I insert the seat post it is generally tight enough not to move by gravity alone, excepting where it is alloy to alloy and quite heavily greased, then it tends to slide down very, slightly / slowly. If it is moving quickly, or dropping straight through, then I'd just get another - quite swiftly.

 

Thanks. I think I'm going to search for a next-size-up post; until then, I'm gonna ride on this with my fingers crossed.