As I am completely new to N, I was wondering if there are any definitive dimensions for stations, platforms, sidings etc.

For instance what length of track would you have for a mainline station platform?

and a branch line station.

For sidings ? coaling and watering sidings etc

I'm planning on using code 55 if that has any bearing on the lengths

Is it better to use the longer points ?

Platform lengths vary enormously for all sorts of historical and financial reasons and were often not ideal. Major stations are urban, where land is expensive or unavailable for example and that is a constraint. There are standards for platform height etc, but many older stations had non standard infrastructure for decades. For most situations longer points will be more prototypical and less trouble.

When it comes to platforms, probably the most important factor is clearance rather than length, as I suspect it is in OO. In N, the humble little outside framed 08 shunter has almost gained legendary status as a tool to test platform clearances!

I use the longest points I can on visible sections of my layout,  for both operational reasons and looks.

I always tend to think the bigger the point the better they look, but am happy to use small electros in fiddle yards so as to maximise the storage length.
I reckon your platform/siding lengths will be down to what traffic you intend to run (Hattons website shows the length of all the rolling stock in their descriptions) but, unless we're blessed with oceans of space, we'll always have to compromise.

Yes, measure a couple of coaches and the platform length needs to be just a bit longer. That will, of course, make a main line station pretty long if you want to accommodate, say, 10 carriages.
If you have lots of room that's fine but otherwise it's best to stick to smaller branchline type stations.
Curved platforms have their own problems of course.
Yes the 08 is great for clearance testing and use the longest points you can to add realism and give smoother less hassle free running.  One thing, I always go on about, don't put a point directly after a curve - stick a straight piece of track in before the point if you possibly can.

My layout design I'm working on is designed to run passenger trains of 1 loco +4 Mk1 or Mk2 coaches. For such consists to comfortably fit in a station, the platforms need to be 76cm long, minimum.
As others have said though, it depends what length of train you want to run, so best way is to put the train together on a straight bit of track and measure it. If, for example, you bought a Class 251 blue pullman 6 car set, that will need a longer station than the one I'll be building, however if your station can fit a 6 car set, then a 2 car DMU will look a bit lost in it...

As a general rule of thumb I take the attitude that prototype station platforms are pretty long things. The branch terminus on my layout can take a hefty long train, but to make up for it on the minus side, the baseboard is only 9" wide at that point! 

The Midland Rly. had a standard platform length of 300ft for small stations. This works out at 600mm (2ft) in N and just holds 6 Mk1 coaches. Smaller non-corridor stock fits more easily. With regard to points, I like to use the longest on the main line and shorter ones in yards, the difference gives a more  realistic look IMHO.

Quote from: steve836 on May 21, 2015, 10:10:45 AM600mm (2ft) in N and just holds 6 Mk1 coaches.

Are you sure? Including the rapido couplings a Mk1 is about 150mm long or roughly 140mm buffer to buffer.

 I agree with the above my platforms are currently aroun 20" long and will accept 3 collet coaches and a tender Loco. I will check measurements later to be sure but deco won't take six coaches.

Also the Pecorino set track plan book has some good info on lengths of various things like coaches and wagons so could be worth a look see

Quote from: Basinga on May 21, 2015, 10:34:57 AM

Quote from: steve836 on May 21, 2015, 10:10:45 AM600mm (2ft) in N and just holds 6 Mk1 coaches.

Are you sure? Including the rapido couplings a Mk1 is about 150mm long or roughly 140mm buffer to buffer.

Just checked, standard length for MR was 300ft , but I cocked up and made my platforms for Cromford 30" (31 1/2 " over ramp) ie 75 scale ft too long and my midland pullman just fits.