There are a number of harnesses and even an escape bag, commercially available and designed for lowering a pet (or a child) to the ground. Your budget, the dog's temperament and your physical ability will affect your purchase decision.
Several things to consider.
The anxiety of your family and your pets is likely to be elevated during a fire emergency. The more you practice, the less stressful it will be. Practicing with the fire alarm ringing as well as in peace and quiet will both be helpful.
Using a pulley system to lower a larger pet will be a good choice. You probably do not want to expose the pet to actually being lowered from the window until an actual emergency, as it may cause undue anxiety (use your judgement). But you will want to practice lowering a weight equal to your pet.
If possible two rescuers should be used: one on the ground to lower the pet (pulley system) and one upstairs and going down the people escape to guide the pet. If your practice time is more than a minute, from bed to everyone on the ground, consider that smoke inhalation is a big risk. Have a plan so everyone does not die trying to save one.
A pet who is scared is going to be more likely to accidentally harm their rescuer with claws or teeth, so devices that enclose the pet may be more appropriate than a simple harness.
If there are lower floor windows below your escape window, anticipate there may be flames coming out of them. Fire alarms that communicate with each other provide an earlier warning, decreasing risks occurring during escape. If the escape path must cross a lower flow window, consider shutters capable of providing a fire block and/or fire appropriate clothing (like firefighters wear).
For small pets, like cats, rabbits and small dogs: use a pillow case as an emergency pet carrier. Take the pillow out of the case and gently slide the pet into the pillow case. Large pillow cases, you can tie a knot in the open end. For small pillow cases, tie the top with rope or clothes line.
This can also be used to lower pets from heights (with the rope) or transporting to the vet. Most cloth pillow cases allow sufficient air circulation for the pet to breathe. Don't use plastic bags or heavy waterproof bags without air circulation.
I have used (long ago) a pillow case for rescuing cats from trees as well as for short transports to the vet.
