For a ship model 3.5' long, the stem should be at or slightly below 3/16" thick measured from port to starboard. However, for a 1:32 scale model of the skipjack Willie Bennet, 1/8" sounds reasonable.
When all else fails (no instructions, no research source info), use the photographs of the model, and take your best guess. You could take a set of dividers, measure a part on the photo that you do know the length of, then measure the fore-aft width of the stem on the photo, and calculate the approximate size the stem dimension by using ratios.
What is a “Strongback”
In the days of large, wooden, sailing ships, shipbuilders would construct their boats on sturdy frames that were very solid, and very level. These frames were typically built out of straight-grained, solid wood timbers and aligned lengthwise down the hull of the ship being built. This large, perfectly squared, and level block of wood was called a “strongback”. A craftsman would build multiple boats on a strongback, always taking care to keep it level and square before starting any new build. Without a properly constructed strongback, any boat built from the shoddy foundation would hardly be seaworthy. Today, strongbacks are often constructed from steel or aluminum, but the focus on sturdy construction and straight and level lines remains just as important as it was in the days of Christopher Columbus.
Do you see those longitudinal strips of wood under the ribs? Those are strongbacks. They act as position holders for the ribs as well as reinforcement to the shape of the hull. Look at pictures in you instructions and see if you can find the wooden members they are calling strongbacks, based on this definition. Some kit instructions really suck. Others are practically non-existent, because many kit makers assume you have some experience building wooden ship models.