I can offer you advise from someone who is a Strida dealer and has helped many people on the Strida for the first time. First the seat mount has a high and low range adjusted by a pin screwed into the seat tube. Strida ships their bikes with this pin in the lower position and that will work for your girl friend. You may need to move the pin to the higher position for your self. There should be no need for a special adjustment.
The frame size is the same on Stridas so when the seat is moved up and down the distance to the pedals changes the same on the LT, SX or the EVO. The difference in wheel size will make it easier or more difficult to put your feet on the ground. How you pedal a bike also becomes important. For myself I pedal with my foot bent so I need the seat higher for my leg length than a person who pedals more flat footed. Because of how I pedal when I have the seat high enough for the correct leg extension I can only get my toes on the ground with 18 inch wheels, this is not a problem for me after I became comfortable riding a Strida. I am about 175 cm. I have seen conflicting information printed on sizing here is a link to Stridas page on the SX, http://strida.com/en/products/?method=detail&aid=199 it states 145 to 193, the same size as the LT model with 16 inch wheels.
I have just sold an EVO with 18 inch wheels to a person about 155 cm tall, she wanted the 18 inch wheels because of the better ride on rough pavement after she tried both the 16 and the 18. She has no problem riding the 18 inch wheels and the bike did not need any special adjustment.
I hope this helps and please ask more questions if needed,
Bill Wilby stridacanada.ca