Project, The wheel size doesn't have much to do with ride height. The height of the tire is what effects the ride height.
The Specials had a 16 inch wheel with a 130/90-16 tire. The figure the height of the tire you divide the width, 130 by 25.4. There are 25.4 mm to the inch. That gives you the width in inches. 5.1 inches wide. Next multiply by the aspect ratio. This is the 90 in the tire size. 5.1 x 90% = 4.59. This is the distance from the bead to the tread. There is tire above and below the wheel so multiply by 2. 4.59 x 2 = 9.18. Add in the wheel size. 9.18 + 16 = 25.18.
The same way with the 18 nch tire. 120divided by 25.4 + 4.72 x 90% = 4.25 x 2 = 8.5 + 18 = 26.5.
Now the axle sets at 1/2 the tire height. 25.18 divided by 2 = 12.59.
The 18 inch is 26.5 divided by 2 = 13.25.
The difference in axle height is 13.25 - 12.59 = .66 inches.
So changing from a 13/90-16 to a 120/90-18 raises the rear axle .66 inches. Under the engine the difference in ride height is half of the rear axle change. .66 divided by 2 is .33 inches difference in ride height. If you figure out the difference between the 100/90-19 front and the 300-21 the difference is less. 3.00 x 90% = 2.7 x 2 = 5.4 + 21 = 26.4.
100 divided by 25.4 = 3.9 x 90% = 3.5 x 2 = 7 + 19 = 26. 26.4 - 26 = .4 divided by 2 = .2.
So you can see that the ride height depending on which way you swap only changes at most 1/2 inch. Going from the 16 inch rear to the 18, the 19 front to the 21 adds the 1/2 inch. Going from the 18 inch rear to the 16 inch rear, going from the 19 to 21 front will lower the bike .1 inches.
To all you hard tailers put what ever tires and wheels you want, it won't effect the stance much if any.
If the bike has 2.5 inches of ground clearance the drop is closer to 3.4 inches. Stock ride hieght, 5.9 - the drop, 3.4 gives the ride height, 2.5. If you have the ride height subtract that from the stock ride hieght gives you the drop.
As James said the rake changing from 27* to 38* will lower the ride height. I don't know the math on that. That gets into geometry.