Thanks to Orbra Bliss, the environment byte for DOS5 COMMAND.COM has
been found! It is at Offset 1566 and the default environment size is
256 bytes.  Many of us patched our COMMAND.COMs in earlier DOS
versions rather than use the /E:xxxx parameter, which often leads to
smaller environments when running secondary processors.

For those unfamiliar with patching, make sure you have a boot floppy
available in case of an error. Copy COMMAND.COM to something like
COMMAND.ORG. Then, at the Dos prompt, if you want a 384 byte
environment, type:
DEBUG COMMAND.COM    <-- you will see a "-" prompt. Type:
E 1666 18            <-- followed by Enter
W                    <-- followed by Enter. You will see "Writing.." etc
Q                    <-- which exits DEBUG

For those preferring a size larger than 384, substitute 20 for the
"18" to provide 512 bytes, 30 for 768 bytes, 40 for 1024, 80 for
2048, etc....

You can check to see that the patch was correctly installed by
typing:
comp command.com command.org <-- or whatever you copied the original to

You will see:
Comparing COMMAND.COM and COMMAND.ORG...
Compare error at OFFSET 1566
file1 = 10
file2 = 18                     <-- or whatever number you entered
Compare more files (Y/N) ?     <-- answer N

Then you can eliminate the /E:xxxx parameter in your Config.sys SHELL=
statement (if you have one). (Don't remove the /P if it's there).