Thank you, Luigi, for excellent snippet! )

\unprotect

%%big 1.5

%%medium 1.25

%%small 1.0

%%

%%bbig 1.75

\def\v!bbig{bbig}

\long\def\OneandSevthirdofthreearguments #1#2#3{1.75}

\letvalue{\@av@\v!bbig }\OneandSevthirdofthreearguments

%%Big 2.0

\def\v!Big{Big}

\long\def\Twothirdofthreearguments #1#2#3{2.0}

\letvalue{\@av@\v!Big }\Twothirdofthreearguments

\protect

Regards, Vyatcheslav