Belgians commonly are between 15.3 - 17.3 hands according to a book I have.
Depending upon the bloodline the horses found commonly in the US are not as heavy a bone build but are taller.
Their European counterpart, known as the Brabart is/was still more a working horse so heft of muscle and bone were what was cherished.
Belgians are the most popular of the draft breeds.
The largest horse in the world currently is a Belgian at 20.2 hands. Big Jake is his name..
Most drafts mature later and still are actively growing at 6 years yet I believe.
Quarter Horses have a big span of common sizes too, between 14 - 16 hands.
Then add in stock, racing, halter bloodlines and their differences and you can see horses near 17 hands.
Maturity also figures in around 6 years.
Between the 2 bloodlines, combined.. the influence is still larger not smaller in stature.
At 17 months of age the horse is just a infant in growing yet...that is true of any horse not just your cross.
Once the horse stops growing "tall" they then start to fill in and grow "wide" as maturing takes place, same as in humans.
It takes years for any horse to finish, truly finish growing and maturing.
The same milestones of growth pattern should be met though before you start to do work with any torque to joints, bones and brain.
All the connecting tissue of ligaments, tendons and muscle also should be encouraged to grow slow and correctly for lifelong health and soundness.
Drafts that I've had contact with were a delight to work with...
Easy going personality, willing, friendly and they wanted to make you happy...
They can make a wonderful riding partner along with a driving/pulling partner in versatility. Enjoy your baby...