I’ve only scratched the surface on learning Sketchup but stumbling along.
I noticed a couple of days ago that my drawing simply wasn’t looking right. I discovered that when I type a dimension of an object in the measurement box that it drew the object twice that size, .5, .5", 1/2" all come out 1". It had been working fine.
I’m on a Mac. I’ve rebooted, run disk utility, etc. even deleted my Sketchup Pro. downloaded a fresh copy and reinstalled it. Any ideas?
Check Model Info/Units and see how you have Display Precision set.
You may have it set 1inch so anything smaller than that will appear to be 1inch.
Also untick Length snapping while you are there if it is enabled.
If it isn’t that attach your model for someone to check.
What tool are you using? If the Rectangle tool, it only creates a Rectangle which is a 2D shape. You cannot enter three dimensions. Enter only two and then use Push/Pull to add the third dimension.
What are you doing to conclude that the sizes are wrong? As @Box pointed out, SketchUp has a distinction between the way values are actually captured in the model and how they are formatted for display. So if, for example, you are measuring after the fact using the Tape Measure Tool or the Dimension Tool and you have model units set to whole number precision, both Tools will round 1/2 up to 1 for display.
I just noticed you are using the web version.
Some of the defaults are a bit ‘rough’.
See here how length snapping is turned on and set to 1" this makes it difficult to draw less than an inch, length snapping should be turned off, it causes more problems than it solves.
Then note the selection of choices you have for displaying your sizes. It is entirely possible to set it up so that everything reads as whole numbers but are actually all different fractions. It is often best to have it set to the finest as it will show errors as you go.
People make the mistake of thinking sketchup is inaccurate because of these settings, the underlying geometry is absolutely bang on what you tell it to be, it is only what it shows that changes. (with the exception of the pointless length snapping which can force a size you don’t want)
By the way, all newer versions of sketchup have an option to weld.
To help achieve your goals, some time spent at the SketchUp Campus and at the SketchUp - YouTube channel will be very worthwhile. Both sites are from the SketchUp team. On the YouTube channel, pay attention to the Square One Series. It covers the basics for each tool.