What size tank would you Tell Janice to put an Oscar and that is the size of a 10-year-old Thumb

When its mature it will need atleast a 75 gallon tank,

I agree at least 55 gallon....i have an 180 gallon tank bit i made it a community tank instead of oscars

You could start him in a 55g but would need to upgrade within the first year to a 75g as they need an the extra width to be able to turn. Oscars typically reach 12" in the first year and eventually grow to about 16".

Agreed. At least 55 gallons.

55 gallons.