Before considering having an artwork framed it is good to have a general idea about the overall size of the finished product you want. There is no point in taking a stamp to be framed and taking home a 6′ monster!
That is an extreme example but hey, hurricanes do occur! In the art world it is common practice to create giants out of smaller artworks. I have seen Miro prints in which the original is no more that a square foot being surrounded by 3 foot of space.
Thankfully this is not the case with most frame jobs. Many times though the client might have an idea of where the artwork will be hung and not adequately consider the surrounding space needed to let a picture be appreciated.
It is always a good idea to consider the hanging context. Many a great master made notes as to the type of frame and colour to use after they completed a master piece.
Mounting a picture
It is common practice when mounting a picture to give 2.5 inches surround mount. It will always depend on the size of the artwork and the larger the picture the more mount space will be needed to find the correct balance of proportion.
The best framing solutions are always based on how the viewers eye reacts when looking at the picture. For example if the viewers eye is drawn to the mount colour before the picture then that will be a framing failure. Equally if the viewers eye is drawn to the mount space first then that too is a framing failure.
Measure your picture
As I mentioned above if you have a space that the artwork must hang in you will need to measure backwards and reduce or enlarge the mount to fit the overall size.
The most important element to consider is to keep the width of the frame and the width of the mount different. If this is not considered then the eye will in effect see two frames with the spaces imitating one another in the minds eye.