I took CD with plans for a sidewheeler to my favorite printers and had full size pond printed+ the article that accompanied them. I knew the plans where for a model with a length of 31.5", which I found a bit small for several reasons. Because the plans were drawn up to use standard dimensional lumber, enlarging them would my require using wood in sizes not commercially available. So that's the dilemma l, I want to enlarge the plans, but will I have to cut a ton of custom sized boards? Oh, and here's a picture of the old girl I'm building.

Enlarging the plan only requires that you provide additional structure for areas that
may now be to far apart for your building material to span. If it had 1/4" frames
in the original plan doesn't mean you need half inch frames at the new scale.
The 1/4" frames can remain, just double the number and try to maintain the spacing.

Everything is constructed to "model scale" you now have.

Thanks Aimee, I understand what your saying,the plans call for 1/4" keel and 1/8" frames,both balsa. In fact, most of the wood in the plan is balsa. I'm definitely going with basswood for most of the major structure though. I think this will help.
As you can see by the picture, there's not much freeboard, another reason I'd like to upsize. I'd hate to be limited to dead calm conditions for sailing her. The details such as window and door frames which the plan uses standard size wood strip could scale up to some odd size that ill have to cut myself.

Tim, you're wise to think about freeboard now, rather than further along in the build when you realize you don't have enough.