From House of Names:

Of all the Anglo-Saxon names to come from Britain, Sizemore is one of the most ancient. The name is a result of the original family having lived at the village of Scarisbrick, near Ormskirck in Lancashire. This place-name was originally derived from the Old Norwegian Brekka meaning hillside or slope and the Old Danish personal name Skar. Therefore the original meaning of the surname Sizemore would be Skar who lived by the hillside.

Sizemore Early Origins

The first dictionaries that apreard in the last few hundred years did much to standardize the English language. Before that time, spelling variations in names were a common occurance. The language was changing, incorporating pieces of other languages, and the spellingof names changed with it. Sizemore has been spelled many different ways, including Scarisbrick, Scarasbrick, Scaresbrick and others.

First found in Lancashire at Scarisbrick, a village and civil parish that dates back to c. 1200 when it was listed as Scharisbrac and possibly meant "hill-side or slope by the hollow." Scarisbrick Hall is a country house located south-east of the village and was the ancestral home of the Scarisbrick family and dates back to the time of King Stephen (1135-1154). The family has held the property since the 13th century, but was sold in 1946 to become a training college.

From Ancestry.com (Sizemore):

English: variant of Sisemore.

From Ancestry.com (Sisemore):

English: from Old French sis ‘six’ + mars, plural of mar ‘mark’ (a coin), a nickname probably of anecdotal origin.

More Reading

• Mystery Explained; the Jewish Sizemore surname