# Size of the box using Manipulate function

I am using the following command:

Manipulate[Graphics[Arrow[{{0, 0}, {a Cos[ω t], a Sin[ω t]}}]], {t, 0, 10, 0.1}]


In order to display a vector in circular motion. However, the box size changes as the program runs, which makes it really difficult to visualize what is happening.

How can I choose a fixed value for the box?

To answer your question literally, you could specify an ImageSize in the Graphics. Note that Animate (as opposed to Manipulate) actually fixes the box size for you. Manipulate is a "stupid" function in that it will just spit out exactly what you have inside it (this is more powerful in general).
What you are probably looking for though is fixed coordinates, via PlotRange:
a = 5; ω = 7;