get a flexible tape measure and start at the cantle and finish at the pommel (following the contours of the saddle)
that will tell you the seat size (if your horse is probs be somewhere around 16-18 ")
then the width you usually measure from D-ring to D-ring
but different makes of saddles with class different widths.
im sure someone will be able to tell you better than me
and seeing as mine was pretty useless... here you go lol
To measure a saddle, you go from the centre of the saddle nail on the side of the pommel to the centre of the cantle in a straight line with the tape taught.