Maximum Dimensions for Installing a PCIe Graphics Card in a Dell OptiPlex 755 Small Form Factor

I use a Dell OptiPlex 755 Small Form Factor computer in my bedroom, and it works great for most things. Though I want to give it a nudge in graphics. But I also don't want to buy a graphics card that won't fit inside the computer. I know I'll need a low profile graphics card. So my questions is, what is the maximum dimensions for a PCIe graphics card in a Dell OptiPlex 755 Small Form Factor.