One option would be to make the graphs auto-stretching. In this type of window you will want to resize the window rather than resizing an individual graph.
New Window( "stretch",
Tab Box(
"Tab 1",
Graph Box(
Y Scale( -10, 90 ),
X Scale( -10, 90 ),
Fill Color( "red" ),
Oval(
X Origin() + 10,
(Y Origin() + Y Range()) - 10,
(X Origin() + X Range()) - 10,
Y Origin() + 10,
1
),
Title( "Oval 1" ),
<<Set Auto Stretching( 1, 1 )
),
"Tab 2",
Graph Box(
Y Scale( -10, 90 ),
X Scale( -10, 90 ),
Fill Color( "green" ),
Oval(
X Origin() + 10,
(Y Origin() + Y Range()) - 10,
(X Origin() + X Range()) - 10,
Y Origin() + 10,
1
),
Title( "Oval 2" ),
<<Set Auto Stretching( 1, 1 )
)
)
)