Size L....which is always the case so far on my ~ 15 year IEM journey. I have yet to find tips that give me seal unless they are size L (or XL). For example, the Spinfit C145 are nicely made, but size L just isn't big enough, and I'm pretty sure they don't make size XL for the C145 at least..
A comparison between Large CP145 vs LL Final E vs Large Sedna Earfit
Another size comparison between LL Final E (top) vs Large Sedna Earfit (bottom).