From: Jo Yardley
But in that other thread there is a lot about size of the file, not really the scale, or at least I can't seem to find it.
Not all of the information in that thread is directly relevant to your question, but certain sections of it are. The final section of the starter post, for example, is entitled "Choosing the Best Texture Size For the Job". That and the section immediately prior, entitled "Allowable Texture Sizes", are what you really need to know.
From: Jo Yardley
If I want to build a wall thats 10 meters high with a image on it that relates to 10 meters in real life, how many pixels should I make it in photoshop?
That's not an answerable question. There's no such thing as "pixels per meter". You can put any size texture on any size object. There's no correlation whatsoever between pixels and meters.
What you should be thinking about is not "size" of the object as measured by units of 3D space (meters), but the amount of screen space the image is likely to occupy. If the item in question is meant to be something that will most likely be viewed from a distance, then it's not going to fill much of the screen for most people. In that case, a very small texture will look just fine. But if it's something people will walk right up to, or zoom in on and examine in detail, then a larger texture may be necessary.
From: Jo Yardley
If I draw a picture of a door on a wall in photoshop that should be lets say 2 meters high in SL, how many pixels should it be?
Again, that's not answerable. Meters have absolutely nothing to do with pixels. A 2-meter door viewed from a hundred meters away might be 10 pixels tall on your screen. The same door viewed from one meter away might be 1000 pixels tall on your screen. How large a texture you should use will depend entirely on how much screen real estate you think the door is likely to cover under normal circumstances.
Remember, the larger the texture, the more of a hit to performance it will cause. For that reason, always use the smallest texture you can get away with for the given task. In most cases for doors, I'll go with a size of 128x256 or 64x128. Occasionally I'll go as big as 256x512, but only if the door needs to be very detailed, and if I'm reasonably sure that people will be likely to spend time examining it at a relatively large size on their screens.
From: Jo Yardley
I just seem to overlook how it works.
Hopefully it's starting to make a little more sense to you now. If not, keep on asking questions. That's what we're here for.

From: Jo Yardley
I would just love to download a image of the internet of someone whos made a nice wall texture for SL so that I can use that to experimente with in stead of making, saving and altering images again and again just to get the sizes right.
You could download a dozen different wall textures at a dozen different sizes. Again, there's no direct correlation between the size of an object and the size of its texture.
Think of it kind of like painting a wall in RL. You could certainly ask a question like "How much paint would I need to cover an 8x10 meter wall?", but there would be no easy answer. We'd have to know an awful lot about what exactly you're looking to do in order to formulate a suggestion. For starters, we'd want to know are you doing a mural or just covering with a single color? If it's a mural, you'll obviously need a whole lot more paint than if it's just monotone. The more details you have to create, the more paint you'll need.
The same concept (loosely) applies to your wall texture in SL. If the wall needs to be very detailed, like that mural would, then you'll need a large amount of pixels to convey the information. But if it's just going to be something simple, then a much smaller amount of pixels will do the job. The physical size of the wall in in-world meters is the least relevant part.
From: Jo Yardley
I really used to be a blonde, I did have more fun then but thats probably because I was 8 years old and I wanted to be a pirate when I grew up and spend most of my time climbing trees and running around screaming

I'm told I was blond when I was a toddler. My mom likes to say "You had the loveliest blond curls I've ever seen." I'm not sure I believe it. My hair is quite dark, and has been for as long as I can remember. But I did love to climb trees and I'm sure I screamed a lot, so we've got that much in common.

I never wanted to be a pirate, though. I wanted to be a starship captain. You didn't happen to want to be a space pirate, did you? You'd better not attack my starship!