Most bows draw weight is measured at 28" unless marked otherwise. The amount of force applied to your arrow that is going to make it bend is determined by the draw weight of the bow at your draw length.
To measure your draw length, draw the bow and have someone mark the arrow on the back side (the side away from you) of the bow or clip a clothes pin on the shaft and slide it down the shaft till you reach full draw, then measure from the throat of the nock to that point. This is your draw length.
Finished hunting arrows should be at least 3/4" longer than your draw length to provide clearance for your broadhead. A little extra length doesn't hurt anything at hunting ranges and can help in the tuning process later. If the bow is marked 50#@28" and you draw 26", your arrow is not going to see 50 pounds of force.
Most bows will increase or decrease about 3 pounds per inch above or below the weight/length it is marked. Now, just because it is marked 50#@28" doesn't mean it is! I have seen them mis-marked by 7 pounds or more plus, if you are drawing the bow into the region of the draw force curve where it is stacking, the draw weight can vary more than 3 pounds per inch.
If in doubt, you need to get your bow weighed on an accurate scale at your draw length. Arrow length also influences how the arrow behaves, short arrows act stiffer during the shot than long ones, even if their deflection on the spine gauge is the same. A rule of thumb is add or subtract 5 pounds of spine for each inch above or below 28", and add 5# if you are using a high performance string.