To draw an Ellipse you have to give major axis and minor axis length.
To use this select
'Insert Menu > Ellipse' or you can also select
icon from
the Entity toolbar. You can easily understand from following figure how
to draw it.

Steps to Draw an Ellipse
Select 'Ellipse' tool from Insert menu or from Entity toolbar.
To give major axis, draw a line (a line with point P1 and P2 )
Then put minor axis length in Inputbar (Or mark only point P3).
You will get an Ellipse with desired major axis( P1-P2) and minor axis(P3-P4) length.