How accurate are bike sizes

Only problem I've ever had was a decade ago with the Canyon online sizeometer suggesting a medium when I've always had large bikes. Going back to it, if I added a cm here and there it would have gone large.

Look at what you've got and compare the geo charts. 

The dimensions chart 'should' be bang on - that's the one with the stack, reach, seatpost length etc. But that only means something if you have a enough knowledge to know what the numbers might mean to you.

Size guides are something else and are at best a guide. If your height is in the middle of the range for a size on their guide and you've not got anything odd about proportion wise or an injury that makes you stupidly stiff, you should e safe enough. But if you are on the margins between two sizes I'd be a little cautious if you've not got much prior knowledge to work with.

To be honest, once you've done your research I'd return here with your key dimensions and the make/model you are interested in and there's bound to be advice available.

Generally they are pretty good.

If comparing numbers to your current bike (which I'm guessing is a few years old if its your only one and you got it second hand) you need to take a lot of figures into consideration, you can't just pluck reach or seat tube from one geo chart and expect that any bike with the same number will fit.

6'3" you should be XL in most if not all brands

Depends a lot on what bike you're buying. MTB / road / gravel. If MTB is it FS or HT? Enduro / DH vs XC / trail?

It's annoying when some manufacturers list an actual frame size and some just go S/M/L/XL. Some also do it on a sort of combined reach/stack measurement.

I know when I bought my last road bike, the size guide put me right on the border of XL and XXL and then recommended the XXL. However I'd ridden a previous version of that bike quite extensively and the geometry remained unchanged in the latest iteration (just the grade of carbon and the finishing kit that had changed) so I was confident to buy the XL - and it's perfect.

I've just been through this. The only definite way to know is a demo, or see if anyone local has the same model you are looking at. Otherwise, it can be worth calling the manufacturer to discuss the sizing if they are the kind of place that answers the phone.

Otherwise, the suggestions above to compare to your current bike - website bike geometry geek let's your directly compare metrics. 

Do you know the "numbers" for your existing bike? Does it feel comfortable/the right size?

If so, use those as a starting point (or look them up on bike-stats.de then use this same website to compare to whichever bike you think you might want to buy -  assuming you're looking to replace your existing bike with a bike of a similar type)

Like you, I'm 6'3" and, usually, end up with the biggest size any given manufacturer has BUT there are some odd manufacturers which must sell a lot of bikes to basketball players as even I find the frames too big! For instance, I tried both L and XL size Giant TCR road bike and opted for the L as the XL was just too big.

I have found the actual measurements to always be pretty good. I ignore sizing in terms of small, medium, large and they are about as good as buying a jumper than is medium or large from different manufacturers as they are all different. Saying that, I would start from a medium and then look at the numbers.

Just to throw another metric out there... The sizing of everything has gone wild since I bought my Meta AM in 2015. 

To give the reach metrics more context, there is a metric called RAD

To give some help, I'm about 195cm probs 196 on a good day, and this helped put the massive 535mm reach of an S6 bike into perspective, as I have a 900mm rad, and the rad of the S6 is 880mm.

I wandered how accurate are the size guides

Variable.
Depending on manufacturer i've been on small, medium and large mtbs. The actual geometry of the bikes was wildly different, despite being in the same sort of point in the market. The L frame i wouldn't even have been able to use a 100mm dropper, and it would have needed a 10mm stem.

Had much the same experience with road frames. I do not need a 56 cm frame.

I think people think about this backwards. The bike sizes (S, M, L, etc.) are a guide for people with average proportions, those are accurate. The problem is that the people sizes are extremely variable. Two people who are both 175 cm tall could have quite different leg, torso, and arm lengths so you may not fit what an average person your height fits. It can also vary depending on the trails you ride and your personal preferences. I prefer a size too small myself, never really got on with very long bikes.