Airtites For 2 Euro - 25mm Or 26mm?

That question scares me. I'm old and although used the metric system in College and work, I still have to stop and remember 25.4mm to the inch. And all the flips I've ever seen at coin shows or coin stores are 1-1/2"x1-1/2" or 2"x 2" or 2-1/2"x 2-1/2". Never seen one but a dealer at a coin show told me there are 3" x 3" ones also.
And so far none at any shows are in the metric system YET.
Good question. I'll wait for someone that knows to answer.

If you are using the ring type capsules they should do fine. Like you said just a little snug on the foam insert. It is the direct fit where you run into trouble.