If you've got a nice picture that you want to display in a frame, but you can find only frames that are too large or too small, you do have some options that will allow you to display the picture without it looking too misaligned. A frame that's too large can result in too much empty space around the picture, and a frame that's too small can depending on the type of frame, simply not allow the picture to fit. Here are three ways to beat the mid-sized frame issue.
If you're dealing with a frame that's too big for the picture, you need to find something that will fill that empty space in between the edge of the picture and the frame. Using only the mat that comes with the frame will make the picture-and-frame combination look poorly planned. However, you can add colored or patterned paper to break up the empty space. Choose wrapping paper or craft paper that has a pattern you like that won't overwhelm the picture. If you choose something in a solid color, make the color complimentary to the picture -- something that matches a light background color in the picture, for example.
Cut and line up the paper so that there is an even border all around the picture. You can fill the entire space with patterned paper if you want, or fill half the space so that it looks like there are two borders around the picture. (That last one works well if you choose a solid color so that it doesn't look like you're just creating another large, empty space in a different color). Fit the picture in the frame and trim the paper if necessary.
Cut the Frame
If you have a wood frame that's too small, and the picture won't sit inside the frame, see if you can cut out part of the back of the frame. You can use a small power tool to remove part of the back of the frame on one or more sides. The front portion of the frame will still be in place, so you'll still have to deal with the edge of the picture being blocked, but the entire picture itself will now sit evenly in the frame.
Get a Custom Frame
Of course, you could also have the picture custom-framed so that the entire thing has the right proportions. While this doesn't have the same immediacy as a DIY project that you do in your own home, it will certainly look very good and not look like a modification meant to hide a cosmetic issue. Custom framing is easy to arrange, and shops have several variations on traditional and modern frames that would suit the picture well.
If you've decided to go the custom-framing route, talk to picture and frame stores (such as Washington Framers' Workroom) to see what they offer. Turnaround, depending on the store's workload, could be quite quick.