First depends on your weight, second depends what fork. The 10mm qr forks, many of then are not recommended with 8 inchers as the leverage provided by a 8 incher can sometimes actually pull the axle out of the drop outs. There was a study done about this and at least the old fox forks ( one's on the old style enduro) made specialized recall the disks on the enduros and put on 6 inchers. Besides, if you use 8 inchers you may be sliding not rolling down the mountain... this isn't good for the trails.
I weight 185 and the 8" rotors are fine (riding juicy 7s) I like them for the added power on descents...be careful though, they are very powerful when being used at low speeds through technical sections, I've had to account for this when riding but I haven't had any problems yet!
In the past i was riding 8'' rotors all the time,i didn't realy care if it was overkill.This year i will be riding 8'' rotors again after riding 6'' rotors last year.A buddy off mine rides 9'' and a 8 rotors for XC.
i use 203mm in front and back with juicy seven's, but I kind of use my bike for do it all, ride xc with it and do a little bit of all mountain with it to, planning to go to a few parks this summer to. and i just put them in because i like having some brake power in my bike and the looks of it.
I have just recently rode the South Downs Way here in the UK, which had lots of very steep, fast decents, and my front 185mm Juicy 5 faded virtually every time. So it depends what type of XC riding you do. If the more aggressive flavour a bigger disc might be a good idea. I am thinking of doing the same myself for next time. There is nothing stopping you using a 185 most of the time and switching to 203 when you think you'll need it.
i use 185mm juicy 5 upfront and 160 on the back and i do pretty aggerssive riding, i reckon thats all youd need unless you were doing something really crazy...
Lets say your XC trail has a FR section to it, big rock faces to good tranny. Having a 203 on the front is confidence inspiring, let alone makes you feel safer.
I'v had a 203mm for a couple of weeks now and the amount of extra confidence it gives you on really fast downhills is amazing. We have just come back from riding the South Downs Way for the second time, (we finnished it this time!) and i was bricking it on most of the fast downhills last time, but this time i was right on the tail of a mate with a full suss, who is a complete nutter! And they didn't fade once The extra weight is negligable in my opinion.
Other thant teh fork limitiations, it totaly depends on the terrain u ride on. On my rides, i have thought about an 8 on the front for sure, and will do it eventually. Weight be damned, but, i am no racer.
Lets say your XC trail has a FR section to it, big rock faces to good tranny. Having a 203 on the front is confidence inspiring, let alone makes you feel safer.
there's just no point in having the 203's they're a bit heavier, cost more and bend easier which will all slow you down in it's own way. but if for some reason you need super stopping power, than just go with the 203's.