I've made a high res here for the Dutch market but it does include a lot of international channels as well as they're available to the market;
http://forum.kodi.tv/showthread.php?tid=228519
I disagree with the current ratio's being mentioned, strongly.
BTW I do have most of these as either .EPS or .SVG so I can make them any resolution or any size I want.
For now I"ve decided on 800x450 which is 16:9.
Why this? For starters not all logo's are square or do fit in a square easily, some are quite long and if this was the case I searched for one which did fit the ratio better.
You see when these get scaled down they become unreadable, let's say if the were 150x800 which happened a lot.
In these cases I've searched for a split version of the logo or a small alternative one.
The ratio of 16:9 did work the best for logo's for me as you can see from the above, I've exiremented a lot with this since the begin days of MediaPortal (about 2001/2002'ish) all the way up to now. Then there's the design... it's really a jungle out there style wise.... I pretty much went with the flat design where available if not all I believe. For this way I suggest a pack rather than a database to make them fit in with each other and the skin. i would support a database but only for let's say I could maintain the logo pack myself for my provider, just as loose logo's I don't see it working out.
Edit: Oh i forgot 1 thing, there's also design; they keep bloody changing them and some channels lost each season!
The pack I made has all the current ones out there.
Pic from my backend for planning recordings etc... Here you can see nicely that square doesn't really fit best and rectangular is better. I agree some logo's could use a bit more space on the bottom or on top but it scales far better overall for all logo's this way.