Snow tires for H Plus Son SL42? I'm riding fixed gear with H Plus Son SL42 rims and Schwalbe Lugano.

I'm thinking about changing my tires to be able to ride in snow/wet roads, but in the H Plus Son site I don't see the maximum size tire that I can fit in the rim.

Given the following diagram, what's the maximum tire that could fit?

I've seen some Schwalbe that look good, but they are 700x30C and 700x33C...
For a complete knowledge base on fitting tires for different rim-sizes, see here: http://sheldonbrown.com/tire-sizing.html
and scroll down to 'Width Considerations'.
30 or 33mm tires will fit your rims but they'll probably need a lot of pressure which might just be counter-productive on snow. On the other hand you should consider another possible issue: 30 or 33mm tires might not fit into the front fork or between the rear stays.
If they do, you might want to buy a second pair of wider wheels and use the wider tires on these. It will keep your 'Summer' wheels nice and shiny.