You can say this, but it's confusing. The reason is that we first hear "one and a half times ..." and we automatically think "larger." Then we hear "smaller." We have to stop, think, and go through a reasoning process much like that in PaulQ's post - while trying to listen to the next part of what you're saying. It would be clearer to say "two-thirds the size."
Plausible, I suppose, although "one and a half times smaller than" suffers from the same problem as three times <as big as/ bigger than>, and that on top of what seems to me to be the illogical combination of "times" and "smaller" generally makes my once-mathematical brain hurt.
Ah, there is a difference between one timeø smaller and one times smaller.
one timeø = one instance; unit of scale; one size; etc.
one times = one multiplied by
Unfortunately we cannot tell whether the times in 'five times' is "multiplied by five" or five instances removed from."
However, as with so many examples of the numeral one, trivial results or mathematical anomalies occur, making 1 a poor example of anything mathematical (except 1 itself.)
Were it "One times smaller" your example's corollary would mean that dividing anything by 1 would result in zero... and to create a negative integer, there would have to be a negative integer (or odd number of them) in the original calculation.
This post was MUCH helpful! In fact I thought of saying "two-thirds the size", or "two-thirds as big", as "one and a half" is always confusing, but thought I should try to stick to the original. Now I realize there is a connotative confusion ('times' suggests something larger in English, unlike in Russian)