There aren't any hard and fast rules for concealment, and no set modifiers based on size. I know that some of the pistols of the era were quite large, though, and relatively unconcealable. My suggestion is to decide for yourself whether or not it's concealable, and set that on your character sheet. A good way to do this might be to include words to the effect in the aspect you assign to the IoP. For example, you might decide to go with "I fight for the land with a Big Damn Gun!" You've now defined your weapon as a Big Damn Gun, and if you were to try to conceal it, your GM could invoke your aspect as a bonus to notice the weapon. This would also justify the +2 One-time bonus, since you've defined the weapon as "reasonably obvious and easy to detect".
Just keep in mind that you'll have to live with this decision, and won't be able to argue that "it's only a pistol, I should be able to hide it easily under this trenchcoat!" No, it's still a Big Damn Gun and will always leave a noticeable bulge -- and the guards will not think you're happy to see them.
As to the ammo question, ammo is kind of hand-waved in Fate, too. Basically, it's treated as a compel against the weapon (the "It is what it is" part of the IoP), assuming it's handled similarly to other Fate games. So the GM will say "I'm pretty sure you've used up your ammo by now" and hand you a Fate chip -- basically you are being compeled to fail your attempt to shoot, and instead spend the exchange reloading your weapon. Since this is a compel, you can choose to accept the Fate and go with it ... or you can spend Fate to refuse the compel -- perhaps the last exchange included a pause in the gunfire long enough for you to duck behind a wall and slap some bullets into your cylinder...