This is a pretty common question, and actually two different points. The basic idea of scaling up the plans seems simple, but it doesn't work so well in practice with the Weekender and the Vacationer. There have been successful versions of the Pocket Cruiser scaled up 12.5% to nearly 18'. We don't recommend this normally, and these were experienced builders. We also would not go any larger with the Pocket Cruiser.
As to scaling up the Weekender, there's no point; use the Vacationer plans! Aside from the fact that the reason we built the Vacationer in the first place was as a larger version of the Weekender, anything in-between is not going to have any more capability than the Weekender really, and the Vacationer plans are already in existence (and tested, which brings up another point: a scaled boat is a NEW DESIGN and needs to be treated as such: through testing and safety considerations abound).
A stretched version of the Vacationer gets problematic for at least three reasons: The existing Vacationer was designed to fit the upper limit of trailering width rules: any larger and one needs special permits. Alternatively, one can make the larger boat skinnier, which we tried. It looks wrong visually, and we didn't like the looks of the stability either. Which brings us to Concern #2: It's very possible that this boat should have some form of righting moment (weighted keel). This is a whole different ball-game from the plans we have drawn. Everything would need to change to accommodate the added weight and higher loads. This is Concern #3: the scantlings (structure sizes) we have for the Weekender and Vacationer are plenty strong at the smaller sizes. Go larger and things get less certain; loads go up exponentially with the volume, not linearly.
All of this adds up to two conclusions: Don't scale up these boats, and if one wants a larger cruising boat project, one should look for an appropriate pre-existing design by someone else.