Another NewBee.

Scale up the ingredients from any of the 5 gallon recipes to around 8 to 8.5 gallons. Then, for primary fermentation you might want to consider using a food grade bucket (of about 10 gallon capacity) rather than your carboys, since the must will need some headroom for potential foaming or cap production (if you add fruit) during the first part of primary, when the yeast are most active. You'll also have a layer of lees on the bottom when primary is finished, and you don't want to keep the gross lees once you move to a secondary vessel. You can then rack to your carboys for secondary fermentation, and airlock them.

you'll want to have as little headspace in the secondary as possible, to minimize the chances of oxidation in your mead.

Welcome to Gotmead, BTW!!

Food grade plastic Brute trash cans make great large primary fementers and you can get them even larger than 10 gallons [the more room, the less the chance of a mead eruption accident (MEA)].

I generally make up my own recipes to suit, so I haven't bothered with scaling others' but it is a simple process. Just take the ratio of the volume that you'd like to make, to that in the original recipe, and apply that as a multiplier to every ingredient in the original recipe. For example, to take a recipe that makes 5 gallons of must and scale it to 8 gallons, multiply every ingredient in the original recipe by 8/5, or 1.6. Since spices don't tend to scale linearly, you'll often see advice by some folks to not use quite as much of a strong spice (like cloves) as the simple ratio would recommend. But it works well for honey, water, fruit, etc. ingredients.