With Graph Builder that is easy by using the size role. I am not aware of a built-in method for resizing individual points in Fit Y by X.
It is however possible using a graphics script.
Example
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
bv = Bivariate(Y(:Weight), X(:Height));
// Create a matrix of rows to highlight e.g.
r= dt << get rows where(:name=="ROBERT");
// Add big markers on top of the points of interest
Eval(
Eval Expr(
Report(bv)[Framebox(1)] << add graphics script(Marker(Marker size(10); Color State("red"), Expr(:Height[r]), Expr(:Weight[r])))
)
);