Brain size is not related to intelligence. This has been known for decades.
However, this is only concerning modern humans who barely live past the age of 70 in the mean. Were we to live significantly longer -- maybe 2 centuries would be enough, maybe more; I'll try doing some maths to get some guesstimates -- it is likely we would no longer be able to form new memories or connections.
There simply wouldn't be enough space in the brain left, both physically and biologically: there's only so much room for dendrites and axons, there's only so many neurons to form them.
In that case, yes, having a larger brain might help, but you'd have to get around the issue of mortality to even bother considering it.