I wrote a long treatise about lineweight and LayOut, but thought the better and offer instead this summary. LayOut is today capable of giving you all the resolution you can use, though we have simplified the UI to keep beginners out of trouble.
On average, the thickness of a human hair is .1mm, In ISO standards, the finest hairline recommended for CAD drafting is .13mm. In traditional drafting, the finest technical pen (ex. Rapidiograph) available is also .13mm. Given the angular resolving power of the human eye (20/20 vision) is about .008deg, the thinnest resolvable line viewed from arms-length is about the same.
The finest hairline you can render with a 300ppi laser printer is .003 in (.0762mm). Similar resolution is achievable on modern ultra-high resolution (ex. Apple “Retina”) displays. You may have a printer that can print 600 or even 1200ppi. Don’t be confused by this unless you plan on viewing your drawings with a microscope. Typically, anything above 300ppi is just an interpolation.
Spare yourself the computational burden of shuffling around bitmap images that are bigger than you need. 300ppi images at architectural sheet sizes are impossibly huge, even if you are opening them in Photoshop. An empty 24x36 bitmap at 600ppi is close to 16mb in Photoshop, even compressed and optimized.
Yes- I regularly read and comment on issues raised in these forums. In fact, the initial builds of LayOut allowed users to choose arbitrarily high resolutions for rasterization in LayOut. I should know, because I designed that feature
In practice, because most folks misunderstand the relationship between rasterization and the human vision system, people habitually set the number way too high and brought LayOut’s overall performance to a crawl. Without really improving the sharpness or clarity of their drawings at all. So we made a change.
I’m sure you’ll take issue with my calculations, but know that before I took on my role as Dir. of Product Management on the SketchUp team, I was a practicing architectural designer just like you. I have quite a bit of practical experience with this issue, and some scar tissue earned by waiting endlessly while too-large bitmaps spooled endlessly to my company’s plotter on the night before a big presentation. For example, here’s me wondering if we would ever finish plotting our competition entry for the Auswärtiges Amt competition in Berlin back in 1995. That’s my boss, Ivan Reimann, asleep on the floor in the background.