Basic handplane theory states that long planes are for straightening wood and short planes are for smoothing it. The planes in the middle can do either job or be set up for roughing out the work.
But all planes do some straightening of the work, and most planes do some smoothing, too. So this simple rule of thumb is actually a bit more complex in practice. After all, some very good woodworkers tend to use one plane exclusively. Paul Sellers favors his No. 4. David Charlesworth is fond of his No. 5-1/2. And the late Alan Peters did most planing tasks with his No. 7.
Here’s my attempt to explain the length of planes in as few words as possible: A handplane can easily and reliably flatten a surface that is about twice the length of its sole. Once your work is more than twice as long as your sole, a good deal more skill is involved.
So yes, you can true a 48”-long edge with a block plane. But it is child’s play to do it with a No. 8.
This rule of thumb also helps beginners choose a good first plane. If you do small-scale work, a block plane or a No. 3 might be all you ever need. But if you build furniture on the scale of Megan Fitzpatrick, your work will be easier with a longer plane.
If you are struggling to get a surface dead flat, pick up a longer plane. If you are struggling to get its surface completely planed in as few strokes as possible (flatness be danged), pick up a smaller plane.
And know that as you get better with handplanes, you’ll usually choose the sharpest one.
— Christopher Schwarz
While my book “Handplane Essentials” is no longer in print, you can still purchase a pdf of the book, which is filled with facts and observations about this iconic tool. Check it out here at ShopWoodworking.com.