Oversize textbooks, rolling backpacks, sub-zero mountaineering parkas: The gear required to equip today's student is getting bigger. But the school locker is not. A typical school locker is one foot wide, one foot deep and six feet high. The dimensions are meant to balance the needs of students and the concerns of school officials, who don't want lockers so large as to hold an entire wardrobe or an entire student. In Montgomery County, Md., middle-school students put the lack of locker space among their top grievances.