Help with Gimp - drawing squares !

[Redrobes]

I know many of you are experts in Gimp and I have an issue that I hope somebody can find a solution to.

In my work I need to draw little black squares on a white background. The thing is I want those squares to be of specific pixel size. So if I start with a blank white image with black as my foreground colour and white as background then how do I draw a 50 pixel square. As in 50 pixels by 50 pixels.

Thats it. Sounds simple right ?

I dont want to have to count the pixels by hand on some zoomed up image. I don't want to create a large black canvas and then crop it to 50x50 and then use that as a brush. I need a fast way of painting those squares then say to change to 30x30 and paint some more and then 85x85 etc.

In my old Paint Shop Pro it was dead easy. When you selected square brush and set it size to 50 it was 50. But its not so in Gimp.

Anyone help ?