The general rule of thumb is that the blade size should be 1/4" smaller than the length of the boot from back to front.
Some skaters prefer to size from the very tip of the boot to the back of the heel to maximize available blade length, and for fast growing children's feet it is possible to "cheat"
a little if a blade still has some life but a larger boot size is needed.
When you are first starting out, a "recreational"
toe pick is just fine. These picks are relatively small, high on the blade and "out of the way"
The pick really has very little purpose until you start to jump and spin. They should NOT
be used to "toe push"
a stroke. This is not only bad form, but it is inefficient and builds a really bad habit that will need to be trained out later.
As a skater starts to learn new skills, 3-turns, spins, jumps, etc., a larger toe pick becomes more necessary. You will notice as blades go up in model, the picks get increasingly large and complex.
The bottom pick is called the drag pick. This is what "scratches"
the ice in a scratch spin. The next pick up in the complex is the master pick.
This is the last thing to leave the ice in a waltz jump or Axel and the first thing to touch the ice on the landing.
Almost all jumps involve the picks in some way. You are generally "vaulting"
from the picks on toe jumps. On the Axel and Waltz you take off from the pick.
Even the Salchow, technically an edge jump, has a little toe pick involvement at the moment of lift off. Look at your imprint on the ice - you will see a little check mark at the point of takeoff.
In stands to reason, therefore, that as you begin multiple rotation jumps you need bigger pick structure. This is why a top end blade costs over $500, while a basic instructional blade is around $50.
For a really in-depth review of blades, including sharpening, and a discussions on topics such as ROH, Radius, side-honing, tapered blades, etc. go to Chuck Wright's FAQ on blade sharpening