I am thinking of buying a Canon XA10 and for that reason I would like to know what the ID of the microphone holder is. To see whether my Sennheiser would fit in. Can anyone tell me that?

It is about 1". Common video microphones such as the Rode NTG-2 (22 mm diameter) or Audio-Technica AT-875 (21 mm diameter) need small spacers on the body to hold them snug. My AT-875 came with O-rings for this purpose and they work great.. I used a few turns of gaffer tape on the NTG-2 body to keep it snug.

Which Senn do you plan to use?

The holder has little rubbery fingers that are about 7/8" ID and will compress to 1" as Don said. I have the AZDEN SGM PDII which is only 13/16" in diameter which is loose but it also comes with a rubber spacer that fills the gap nicely. Cheers

My Sennheiser is 22mm as well, so I will need to acquire a couple of O-Rings. No big deal.
Thanks for the input.

I have a Senheiser mic I use in mine. You will need to add something to make it tighter. I have a thin piece of soft rubber I wrap around my mic one time and it works great. Mics vary in diameter, I'm sure they designed it to accept the larger popular mic sizes. I've needed to do the same thing using Senheiser mics on other cameras.

I have used this multiple times and it holds well.

My Sennheiser fits fine. I have a thin piece of rubber I wrap around it. My Sony Z5 requires the same piece of rubber too.