I have knitted a fair Isle pattern for years when my children and grandchildren were young. Nor my great grandchildren want the same pattern. So far I have knitted all sizes.
The next one I knit will be bigger than the largest size on pattern. Is it easy to adjust a pattern for a bigger size, bearing in mind it is Fair Isle

I’m not sure it’s easy to adjust but since you know the pattern so well, that’ll certainly help. You’ll need to keep the fair isle pattern repeat in mind so that there’s no break in the pattern. As long as you adjust the stitch number and lengths to stay in proportion, you should be fine.

What pattern are you using? Can you give us a link or a pattern name (don’t print the instructions here as that causes copyright problems).

You might check out some similar fair isle patterns on Ravelry in the size you need, just to get an idea of dimensions.

Patons pattern 7285

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Maybe this one?

How much larger to you have to make this?

Just two sizes

OK, that’s probably too much larger to use a heavier weight yarn with the pattern in order to increase the size.
You’ve certainly got the motivation to get this to work. Checking out similar projects with the same yarn will give you a guide too. Good luck with it and post a photo when you finish!