There are several ways of tackling this issue.
The first is to scale the recipe back in volume. To do this you will need to use the 'scale recipe' function and reduce your ending volume until it fits within your equipment constraints
The second thing you can do is to reduce the gravity of the wort, thus requiring less grain. The easiest way to do this is to click on the OG slider below the ingredients window on the design page. You can decrease the OG value in the box until the recipe will fit your equipment.
The best way (always saved for last), would be to add DME or LME into the recipe to be added during the boil and back off on the base grain accordingly. Here you can reduce the base grain until you have reached your 16 lb maximum then add the appropriate extract into the recipe and increase it until you get back to your original target OG.