Is there a volume limit above which you might not offer standard chemoradiation for a patient with a GBM?

I think this requires more information to answer. I presume the question is if there is a GBM that is of a certain size for which I may not offer standard chemoRT. There may be situations for which I may not offer standard 60 Gy in 30 fx IMRT and consider hypofx RT including older patients or patients with poor KPS. I may consider omitting RT altogether if an older patient, with poor KPS has MGMT methylated GBM who I don't think would tolerate fractionated RT. In some patients with multifocal GBM or gliomatosis for which RT may be more like WBRT, I would discuss the pros and cons of chemoRT vs best supportive care depending on the patient's wishes.