Would radiation treatment work on a 5.9x4.1x4.2cm left frontal lobe meningioma (benign or malignant)?

Unlikely : Most meningiomas are benign and radiation is not indicated for treatment. If the meningioma is malignant, then radiation could be indicated after surgical resection.

Absolutely. : It can sometimes be best to use surgery on an easy to remove lesion, but if not easy to remove radiation can work. My bias is to avoid treating normal brain as much as possible, so we use proton therapy for this sort of lesion: if it cannot be removed with surgery. Many types of rt can be used: protons, gamma knife, cyberknife, ig-imrt, vmat, etc. Sometimes observation is best.

A large tumor: A tumor of this size may be causing symptoms and serious consideration of surgical resection should be undertaken by a skilled neurosurgeon. Only if an excellent surgeon has recommended against surgery or the patient has serious medical problems would I advise radiation treatments in a large tumor like this.