Preparing road bike for a long trip I have the triban 520 C1 bike, size M, I'm 168cm high. I want to ride a 1000km brevet on this bike (divided into 150km-200km parts). 

I already changed a saddle into the one from my old trekking bike, so it's much more comfortable than the default one. I also took M sized one instead of S so that it's more comfortable.

Ok, first things first - tires. Right now I have 25mm, these is the stock width. Should I change them? To 28mm, or 32mm?

Second - panniers. Can I put them on this bike? Which ones? What would be the impact on the comfort of riding?

How can I prepare my bike to make it?
Okay, so this is an aluminium-framed Sora road bike, with a carbon fork, from a European chain store, as shown here?

First of all, there should be no problem doing a 200km ride on that type of bike, its what its made for. And, you should be fine to do several of these rides in close succession. The bike doesn't have the greatest drivetrain, but for the rides you're talking about, it should be acceptable. So, other than generally making sure the bike is roadworthy, no special preparation is required.

You seem to be asserting that a larger frame will be more comfortable than a smaller frame. I'm not sure where you got that from, but it isn't necessarily true. The comfortable frame is the one that fits. That said, this bike looks like a compact frame so there's probably quite a large range where the frame would be considered an "acceptable" fit.

For road riding, 25mm is a perfectly reasonable tyre width. Try them out. If you think you feel too much of the road when you ride, a wider tyre will give you more of a cushion, but you need to make sure that the rims can take a wider tyre, and that you have clearance between the new tyre and your brakes/frame.

For your panniers, in your place I would contact Decathlon and ask them whether the bike will accept a rack. But be prepared, there is every likelihood that they will say no, and for sure I'd be very wary of trying to put a rack onto an aluminium frame (or front panniers onto a carbon fork, for that matter). But see what they say. If the bike can have a rack fitted, Decathlon will, I'm sure, be able to recommend something out of their range to you.