Woodenboat mag has published several patterns over the decades for the price of the issue. Glen-L and other have them for sale. Google will obtain several free patterns in pdf format. Pete Culler and others have books out on oars with patterns and instructions.
Most involve plotting offsets from a centerline. Fold craft paper down the center, plot your offsets using an awl that pierces the paper, then fold your paper, connect the dots using scissors and you have a perfectly symetrical pattern that can then be stapled to and transferred to posterboard or doorskin.
Oar length is easily adjusted by drawing the blade and handle seperately along a centerline of any length and connecting/fairing the lines. Scantlings are determined by the pattern and wood species used. Most builders make them too heavy.
More oars....these are 8 and 9-footers made in one long day after glueup:
http://pic20.picturetrail.com/VOL12/.../100386741.jpg
http://pic20.picturetrail.com/VOL12/...9/91841741.jpg
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http://pic20.picturetrail.com/VOL12/...9/93859833.jpg
http://pic20.picturetrail.com/VOL12/.../101687783.jpg
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