"Vossa", "Vosso" is basically the same as "Sua", "Seu", however it is extremely formal and rarely used in Brazil. "Vosso/Vossa" is the possessive for "Vós", the third person plural, and "Seu/Sua" is the possessive for "Ela, Ele, Você", the third person singular.

Ele tem quase a sua idade. <almost>

Why is quase (almost = duolingo definition) used and not sobre?

So if you want to say about in relation to measurement, tem mais ou menos or quase are the two main ways of saying it?