ヘレンはせいぜい４０キロだ。

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

at the most; at best; to the utmost; as much (far) as possible

four

circle (sometimes used for zero); 'correct' (when marking); symbol used as a placeholder (either because a number of other words could be used in that position, or because of censorship); period; full stop; maru mark; semivoiced sound; p-sound

kilo-; 10^3; kilogram; kilogramme; kilometre; kilometer; kilo-; 2^10

be; is; indicates past or completed action; indicates light imperative