I played with a 13mm for years as it seemed to be the standard size tip for new cues. However I found when I got to an 11.75 that I had more action on the cue ball. Actually it just seemed like you had more options on the cue because the 11.75 seemed to be more of a precise hit.
I guess that is why some say play with a 13 and get good and then move to a smaller size?
However, if you learned to play with a 11.75 LD shaft then you wouldn't have to make that adjustment when you got better now would you?
I think like Scott says it is just all preference. Find out what works for you and what feels good to you. Honestly this can and will change over time. Many years down the road what you started with will be something different that what you choose to play with when you get better.