Ok, check out size reduction of files when saved as jpeg. Even on zero compression. It's quite a huge reduction. There's nothing to be gained in sending in files larger than the required physical size at 300dpi, so there was some reduction there in this case if I recall correctly.

Generally I paint "a la prima" digitally, so I don't use layers for any length of time. That really helps keep file sizes manageable. Not much help if you want to use several though I realise!

Regretfully I can't really help much to figure out how to do big files on a particular spec machine, or even a PC. I've invested in the best Macs I can afford for ten years or so, as a necessity for my job, so I'm a bit limited in my experience there! Keeping layers to a minimum I have already mentioned, but yup, each individual machine will give up at a certain point, so you have to work within sensible limits that are right for your hardware.

I print all kinds of images, and am extremely happy with the colour repro. So it's not to do with colours, or lack there of, in a given file. I'll pull out some more brightly coloured ones tomorrow to show.

Delivery from Posterxxl is a very good question. At checkout they say prints are despatched within... Now I can't remember off the top of my head how soon, but that is slightly misleading, since it usually takes ten working days or so. Which is fine once you know and can plan accordingly.

Something that works undeniably in my favour, but sounds a bit pompous, is years of experience working in digital paint for print reproduction. That's something that is hard to impart in a few sentences, but generally you can't just paint anything and expect it to print well. Screens of course make colours very differently to inks, and working within the printable range of colour and tone is something that takes practice. And that final stage of adjustment in PS. Really saturated colour will usually fall out of "gamut": strong screen reds, cyans and greens can all pose problems for reproduction in ink. Making things a little lighter, a little less saturated, boosting tonal contrasts where necessary... All that stuff really helps get a good result.

Of course you also need to know your monitor is well calibrated too, and I've already typed too much on this tiny iPod keyboard!

Again, I hope there's some useful info in there somewhere! Adjusting digital images for faithful reproduction is a major skill set in itself.