With the 18 inch tires for the strida we have very little choices in order to fit the frame.
1)schwalbe kojaks 18x1.25
2)schwalbe marathon plus 18x1.35
3)strida innova tires 18x1.35
if you choose not to use fenders at all you can squeeze in the Marathons 18x1.5 but it will be extremely tight and might rub the frame.