An oocyte is an immature egg cell of an animal ovary. During human menstrual cycles, one oocyte matures to become an ootid which then becomes an ovum. While this happens several other oocytes mature but then disintegrate (1). Oocytes are massive compared to other cells in the body, they have a diameter of around 100 microns (0.1mm) (2). Their diameter is roughly around the same thickness as a single strand of human hair) (3).
(1) Dictionary.com, 2018, oocyte definition, viewed 21st October 2018, 
(2) Bruce Alberts, 2002, Molecular Biology of the Cell, 4th edition, viewed 21st October 2018, 
(3) CBS News, 2018, Human eggs; 9 fascinating facts, viewed 21st October 2018,