Just wondering what length and weight of a rod you would use for dry fly and nymphs on a lake

Unless particularly long casting is required I would use a rod of between 9 and 10' AFTM 5 or maybe 6.

If longer casts are required you might be tempted to go up to a 10' #7 but you would need to be careful as small flies require lower diameter leader points to present them properly and this combination might be a little too powerful especially if the rod is fast actioned.

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“There is no more lovely country than Monmouthshire in early spring. Nowhere do the larks sing quite so passionately, as if somehow inspired by the Welsh themselves. There is a blackbird on every thorn and a cock chaffinch, a twink as they call him there, on every bush...... It moved me profoundly. I had been spared to see another spring, and I thank God for it.”

Oliver Kite
“A Spring Day on the Usk”
A Fisherman’s Diary

I do almost all my fishing from boats and Ive been using a 9'6" #5 weight outfit for as long as I can remember for fishing dries.I prefer the sensitivity of light tackle for this kind of fishing but I know plenty of people who are perfectly happy with #7 weight ten footers.