The wood for the blanks is selected to be of premium quality and specifically appropriate for bow making: high density, straight grained, free of knots and imperfections, and quarter-sawn (i.e. the grain runs parallel to the length of the bow).
All blanks are symmetric (with the handle block located in the geometric centre of the blank) and cut on a straight-taper from a maximum thickness at the handle to a minimum thickness at the tips. This pre-tapering will save a considerable amount of time in the tillering process whilst still leaving you with the requirement to hone the skill for yourself.
As a rough guide, 4ft blanks can be expected to reach a maximum weight of 20lbs @ 20”; 5ft blanks 25lbs @ 28”; 6ft blanks 40lbs @ 30”. These weights are not guaranteed, however, and much will depend on skill levels and the specific, unknowable properties of each individual piece of wood.
For those who would like a recommendation for a video tutorial / build-along, it’s hard to beat this from Dan Santana.