background-image: -moz-radial-gradient(50px 100px, circle farthest-corner, #ffffff, #ff7ae9 200px);
background-image: -webkit-radial-gradient(50px 100px, circle farthest-corner, #ffffff, #ff7ae9 200px);
background-image: -o-radial-gradient(50px 100px, circle farthest-corner, #ffffff, #ff7ae9 200px);
background-image: -ms-radial-gradient(50px 100px, circle farthest-corner, #ffffff, #ff7ae9 200px);
background-image: radial-gradient(50px 100px, circle farthest-corner, #ffffff, #ff7ae9 200px);
In this example the '200px' is the size of the circle, any standard
CSS units such as px, em or percentages are fine.
The '50px 100px' is the position of the centre of the circle, it works
the same way as background-position so values like 'left top' are
fine too.
There are a few online generators that can help you with all the
vendor specific prefixes.
p.s. @Mohsen pixel values are fine, MDN says:
either a percentage between 0% and 100% or a length along the gradient axis
If you click on 'length' it says
The CSS syntax for length is a number followed immediately by a unit. Space between the number and the unit is not allowed.