The majority of the time if you are looking for a frame size, the size of the frame will be determined by the size of the artwork you will be framing. Example, an 8 X 10 photo would be put into an 8 X 10 frame. This would be considered the inside measurements of the back of the frame opening, not the front opening. This information would be the same for custom order frames as well. You need to measure the length and width of the item being framed before purchasing a frame, so you do not have a frame that does not fit your artwork. This happens more often with custom frames.
If you have a small item or photo and want a larger frame presence on your wall, you can mat the item and place it in a larger frame or purchase a frame that has a wider moulding size. When matting a picture or artwork you, will want to use the overall size of the mat to determine the size of the frame you will need.
When framing needlework you will want to have your needlework piece mounted before you purchase a frame. The size before mounting is not the same as the stretched size. Some pieces of needlework needs to be stretched over stretcher bars or stretching strips. Some needlework needs to be laced while others are mounted to a sticky board. Any of these mounting techniques will depend on what kind of needlework you have and will determine what kind of frame you will need. Measure the size of the mounted needlework in order to determine the correct frame size.