How do I say: Something is 1/1.5 times as big as something else? e.g. using the analogy X is 1/3 (one-third) as big as Y

Here is my challenge:

According to the proper grammar of English:

My house is TWICE (2X) as big as yours.....Your house is HALF (1/2) as big as mine. CORRECT.

How do I then say that:

My house is ONE AND A HALF (1.5X) times as big as yours.....Your house is 1/1.5?

My house is (2x) twice as big as yours.

My house is (⅓) a third as big as yours.
My house is (½) half as big as yours.

My house is (1⅓x) a third again as big as yours.
My house is (1½x) half again as big as yours.

The use of again after a fractional expression is used to represent that something is of equal size plus the fractional number.

This is the sense of again that's being used:

[Merriam-Webster]
4 : in addition : BESIDES
// Again, there is another matter to consider.

The use of again can come after any fractional representation.

Outside of the particular phrasing in this context, you might hear something like this:

Give me a pint, and then half a pint again.

To paraphrase and analyze the context of its use in house sizes, it means the following:

My house is [the same size] as yours and [this fraction] bigger [in addition].

The use of half again specifically is common enough that it's actually become an idiom:

half again as much/many as
Merriam-Webster
US
—used to say that the size or amount of one thing is 50 percent more than another
// If 100 people were expected and 150 came, half again as many people came as were expected.
// This dress costs half again as much as that one.

Note that the definition includes US, so this could be less common in the UK.