New sounding rod

Are you worried about diameter or length? With diameter, take your time. My guess is that 6 or 8mm is a good starting diameter for most people, then get gradually larger toys (or one with gradually increasing bulbs or rings) to work yourself up. I have a lot of wonderful toys in the 9.6-10mm range and don’t really want to go larger and lose the ability of those to stay in. But some guys swallow 13mm or more, all depends on your anatomy and internal size. You don’t want to be tearing your internal tissues, though. Size and pleasure aren’t necessarily related.
As to length, start with 2-3 inches and again, increase. My largest rigid plug is about 9 inches, and I have to be COMPLETELY relaxed (not erect) to get that in. Takes some practice to get used to the plugs so you don’t quickly sproing to an erection! Length doesn’t give you a better or stronger orgasm, but it is fun to massage deeper inside yourself. As with the diameter issue, best to explore when you’re not rock-hard. The last thing you want is to poke a hole in your internal piping. Looks like your plug has a nice rounded tip on it – that’s really good. PS – use LOTS of lubricant on the plug. Also fun to use a “lubricant launcher” to inject some Surgilube into your urethra in addition to slathering the plug itself. I use about 1/2 inch of lubricant in the tube.