This might be extremely convoluted and I might be missing the obvious, but without OpenPyXL filling in the column_dimensions in Iterable Worksheets (see my comment above), the only way I can see of finding the column size without loading everything is to parse the xml directly:
from xml.etree.ElementTree import iterparse
from openpyxl import load_workbook
wb=load_workbook("/path/to/workbook.xlsx", use_iterators=True)
ws=wb.worksheets[0]
xml = ws._xml_source
xml.seek(0)
for _,x in iterparse(xml):
name= x.tag.split("}")[-1]
if name=="col":
print "Column %(max)s: Width: %(width)s"%x.attrib # width = x.attrib["width"]
if name=="cols":
print "break before reading the rest of the file"
break