Bought a frame one size larger, how to fit for slightly smaller rider? I've recently bought a frame that is about one size larger. It's a 22" frame and my height is about 5'9".

I had a choice between the 22" frame and an 18" frame so I chose the frame that was a bit larger because I had a bad experience on a frame that was too small.

What are the potential downsides to a larger frame and could it be fixed with adjustments such as a lower seat post.
Think of bike fit as a triangle, looking at a rider on a bike from the side, with a point of the triangle at the hands, a point at the saddle sit position, and one at the bottom bracket. Now with the bottom bracket position being fixed, it isn't always possible to replicate the exact angles and lengths of the triangle from one bike to another, but between one size and the next, you usually can get close enough to be considered equivalent. 

You will want to lower the saddle height, adjust the saddle forward/back slider more forward towards the hands, and you will want a shorter stem, perhaps with a modified angle to get your hands where you want them. You should also consider the length of the crank arms, as sometimes manufacturers will configure different crank arm lengths for different size bikes. Most bikes are sold with longer than ideal crank arms anyway, and should be one of the first components considered for replacement. The main concern with frame size (aside from stand-over height), is the angle of the line drawn between the bottom bracket and the saddle sit position, as that affects the angles and forward/back position of the knees throughout the pedal stroke, and also where the rider's center of gravity is centered between the front and rear wheels (which is a primary input to how the bike handles or feels). A secondary consideration is the stem length, as too short and it will negatively affect steering characteristics. 

Most will prefer a smaller frame configured larger rather than the opposite, with the added weight due to more frame material of the larger bike often cited as a complaint. I'm riding an undersized frame presently, with a lay-back seat post, and I look forward to having a better fitting bike someday where I don't feel so much like I'm sitting right over the rear wheel. It feels a little too twitchy; having the center of mass more between the wheels makes it feel a little more like you are riding in the bike rather than on top of it.