I don't believe there is any limit on the electrode dia, since you are writing a new procedure, but you can't take an old procedure that lists a certain size and increase the dia more than 1/32" per Table 4.5(10).
However, I would think hard about using a really large electrode for the root pass, as that groove is kinda narrow and you may not get good penetration at the root, plus if the electrode doesn't fit down in the groove to the root, you will have porosity issues. I would write it to include a smaller dia for the root pass, and step up to the larger dia for the fill and cap passes. I believe you can do this and still remain prequalified unless I'm missing something.