Sizer may refer to:

• Sizer Barker, an indie band from Liverpool, England

• Stephen Sizer, English author on Judeo-Christian topics
• Ted Sizer (1932–2009), founder of the Essential school movement
• Theodore Sizer (1892–1967), American art historian, author

• Mount Sizer, a prominent peak located on Blue Ridge in Henry W. Coe State Park in California
• Crusher, equipment to break and reduce the size of rocks, coals, overburden or other minerals