In reply to Pradyumna Panda:
Maybe it's easier to see if you flip your description to more closely match the antecedent -> consequent structure of SVA.
// when AWVALID is asserted and AWREADY is LOW, then AWSIZE remains stable
AWVALID && !AWREADY |-> ##1 $stable( AWSIZE );
The only thing that might trip you up here is the ##1, because $stable is backwards looking, not forwards looking.