Somewhat clumsy but you can use the Windows calculator or other means to get the radius of a circle in your map units needed for 6 acres (e.g. square root of 43560*6/pi for map units of feet), or the length of a side in a square (e.g. square root of 43560*6), and copy that to the clipboard. Then either use the circle graphic or the circle tool on the Advanced editor toolbar and start drawing your circle and while doing so hit the r key. Paste in the radius and you have your 6 acres.
I cannot find an equivalent method for a square unfortunately, but with a square graphic you can open its properties, and on the size tab paste in the lengths. You could convert that for a square shape or you could use the cogo tool or the sketch tool (right click and use the direction/length tool) to paste in the needed distance and angles. Copy and paste if you need a few more.
Or you could create a vector grid (e.g. Hawth's Tools) using the calculated square's line lengths and then just select out as many as you need and delete the rest.
