BUG: Illustrator auto-changing size

Do you need to select preview bounds in the preference setting as well? I use inches....and none of my sizes stay set as desired

Thanks for catching that – I changed my answer.
It still is a bit mysterious to many users when/where Illustrator sets the default for new items, especially with the new Properties panel.

I even think there's a bug there. E.g. when the Align to Pixel Grid and Pixel Grid Preview are set ON, clicking with a rectangle tool anywhere in the artboard, setting the shape to be 30 by 30 mm (line width 1 pt) should at least display in an anti-aliased manner. But the rectangle appears crisp and sharp, while the Properties panel tells the size is 30 x 30. Hitting the Align to Pixel Grid button changes the sizes, but the display doesn't change. Strange...

Which version of Illustrator are you using and are you using a web document profile for new documents??

Daniel,

Reading this thread earlier, I was certain that Ton had given the answer, but

and only with the width.. with Height I don't have the same issue.

has made me believe that you may have some kind of (subtle) corruption.

And this leads to the list as a possible way out, unless it is just happening in one document (see A)):

If things that should work simply refuse to (all possibilities exhausted including the A) - C) below (and carefully making sure you are performing (all) the needed things)), you may try (the relevant part(s) of) the list 1) - 8) below.

Sometimes, (certain) things may fail or stop working for no apparent reason. When the (other) possible reasons/cures fail to work, it may be some kind of (temporay or permanent) corruption, or even some inconvenient preference setting(s), which may be cured with something on the following list set up in an attempt to provide a catchall solution for otherwise unsolvable cases. It starts with a few easy and harmless suggestions 1) and 2) for milder cases, and goes on with two alternative ways 3) and 4) of resetting preferences to the defaults (easily but irreversibly and more laboriously but more thoroughly and also reversibly), then follows a list 5) of various other possibilities, and it ends with a full reinstallation 6). If no other suggestions work, or if no other suggestions appear, you may start on the list and decide how far to go and/or which may be relevant.

The following is a general list of things you may try when

A) The issue is not in a specific file,

B) You have a printer correctly installed, connected, and turned on if it is physical printer (you may use Adobe PDF/Acrobat Distiller as the default printer with no need to have a printer turned on, obviously you will need to specify when you actually need to print on paper), and

C) It is not caused by issues with opening a file from external media.

D) It is not caused by other applications (you may close down/disable everything else running, including browsers and antivirus, then try again). There are quite a few usual suspects that may disturb and confuse Illy (job description Adobe Illustrator) so she moves in mysterious ways (even more than usual, some would say).

You may have tried/done some of them already; 1) and 2) and 3) are the easy ones for temporary strangenesses, and 4) and 5) and 6) are specifically aimed at possibly corrupt/inconvenient preferences); 5) is a list in itself, and 6) is the last resort.

If possible/applicable, you should save current artwork first, of course.

1) Close down Illy and open again;

2) Restart the computer (you may do that up to at least 5 times);

3) Log out of your Adobe account and log back in;

4) Close down Illy and press Ctrl+Alt+Shift/Cmd+Option+Shift during startup (easy but irreversible);

5) Move the folder (follow the link with that name) with Illy closed (more tedious but also more thorough and reversible), for CS3 - CC you may find the folder here:

https://helpx.adobe.com/illustrator/kb/preference-file-location-illustrator.html

6) Follow the instructions here: https://helpx.adobe.com/illustrator/using/setting-preferences.html

7) Look through and try out the relevant among the Other options (follow the link with that name, Item 7) is a list of usual suspects among other applications that may disturb and confuse Illy, Item 15) applies to CC, CS6, and maybe CS5);

Even more seriously (this may be serious because you may need to restore plugins and whatnot afterwards if you have customized things), you may:

8) A) Uninstall (ticking the box to delete the preferences if applicable), B) run the Cleaner Tool (if you have CS3/CS4/CS5/CS6/CC), and C) reinstall. You may try without step B), but sometimes it is needed, because otherwise things may linger.

As it appears from this thread, it may be worth repeating this if it fails to help the first time:

https://forums.adobe.com/thread/2283530

To uninstall:

https://www.adobe.com/cfusion/search/index.cfm?loc=en_us&term=uninstall&cat=support&product=illustrator&self=1

Cleaner Tool:

http://www.adobe.com/support/contact/cscleanertool.html