A report Table Box cannot be directly resized (you can resize columns, see this post), though you probably just need to turn off the scrollbars so that the tablebox is as big as the content. I'll illustrate here:
Names Default To Here( 1 );
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
jmp_report = New Window( "Big Class",
Text Box( "Big Class" ),
H List Box( Outline Box( "Big Class", tbl = dt << Get As Report ) ),
);
tbl[table box(1)] << Set Scrollable(0,0);
-- Cameron Willden