Section 13.2, The edm:ValueTerm Element, in CSDL allows a Type attribute, but does not mention property facets. csdl.xsd allows a MaxLength attribute in a ValueTerm element, but not a FixedLength attribute.
Allowed:
<ValueTerm
Name="ContentType"
Type="Edm.String"
MaxLength="Max"
Nullable="false" />
Disallowed:
<ValueTerm
Name="ContentType"
Type="Edm.String"
FixedLength="20"
Nullable="false" />