Willem is right, there's a fair amount of math to do.
You won't get the full 2" advantage going from the Seca 18" to the Maxim 16". The Maxim's wheel takes a larger tire so the actual tire height may not be much less.
Also, if I am not mistaken, the rear sprockets will be different. My Maxim has a 35T and I believe the Seca uses a 37. I had always assumed this was to adjust for the 6-speed tranny (Maxim is 5-speed), since the rpms in top gear are the same. But maybe it adjusts for a rolling circumference difference too.
You might also have a problem swapping to a 19" Maxim front tire as the brake disc is offset on the Maxim but flat on the Seca.
If you aren't riding in heavy duty motorcycle boots yet, you might want to consider sinking some of your funds into a pair. The thick soles and heels make a noticeable difference to your leg length and footing. I am a touch vertically challenged myself and I love my used set of Harley riding boots for that very reason. Well worth the $60.