Measuring Projection = Measure the distance you want covered from your house to the end of the covered area.  The Posts will generally be set directly under the Header/Gutter.
Measuring Length = Measure the distance you want covered along your house.  The Posts will be generally set back about one (1) foot from the end of the covered area under the Gutter/Header, Post spacing will vary depending on Snow Load, Projection and Height.  You will need to fill out a Free Quote to have the details figured on your particular application.

Note: If you are attaching to your Fascia only measure the area that the awning will be covering.  Do not include the Eve/Fascia projection on your house, only measure from the point where the Awning will be attached to, Eve/Fascia attachments should be measured from the outside of the Fascia board.
  For example: You want to attach to your Eve/Fascia of your house and have a 10' slab coming from the wall and you want to cover the entire 10' of slab.  To measure properly you would measure from the outside of your fascia board to the end of the slab.  Say you measured your Eve/Fascia projection and came up with 2' in projection.  You will want to order an awning with an 8' projection to obtain a total distance away from the wall (not Fascia) of 10'.

If you are going to be attaching your awning to the eave / fascia then you will need to take three (3) measurements for us to let you know if the engineering will work so you can obtain a permit.
1) Rafter Spacing on Center:  Measure from the outside of one rafter to the inside of the next rafter to get your spacing on center; usually rafters are spaced every 16 or 24 inches.
2) Rafter Size: Measure the width and height of your rafter, usually rafters are either 2" x 4" or 2" x 6".
3) Eave Projection:  This is the distance out from the wall to the outside of the fascia board.  When you measure this do not follow the angle of the rafter, you will want to measure straight out from the wall using a level.

Note: When attaching to the fascia you will need to place a 90 Degree clip from the rafter to the fascia board.  This clip is commonly known as an A-34 clip, you will have to purchase this locally for very little expense at any Home Center.  This may be a problem for you if you have a closed soffit, you have to be able to get into the back side of your fascia to install this clip or you will not be able to meet engineering and obtain a permit.