??? Fuse UpGrade??What Size???

 In an effort to maximize the potential of my SimAudio Moon 240i integrated amplifier I have decided to try one of the Synergistic Research Purple fuses...
 The owners manual states it uses a 4 amp,Fast Blow,Type AG fuse..
 However,there are 2 sizes of this fuse offered,5mm x 20mm & 6.3mm x 32mm..How do I determine which size I need? Pulling the old one out & measuring it comes to mind but I was hoping there is a faster way to determine this...
 Thanks very much for any assistance..