You can use a double layer DVD – DVD+R DL
Up to 8.5GB

DVD-R and DVD+R hold about 4.2GB or a bit less after their formatting by the burning program and the DVD+R/DL holds about 8GB, it’s always less than the packaging indicates.  I use the DVD+R/DL when downloading the .iso file that contains both the x86 [32-bit] and x64 [64-bit] versions. 
The Bootable USB drive has similar limits, best to use 8GB for the x86 or x64 and a 16GB for the combination. 
I get the latest/twice yearly Versions using the MCT/Media Creation Tool, haven’t had a problem yet.  And I watch various sites to catch when some of the Build updates are added to those downloads, great for doing offline installs and also for Upgrades when Windows is still running, not needing the booting.

dhdoyle wrote:

I’m still running 1803 but I grabbed an iso of 1809 for the future. The X64 version won’t burn to a DVD. I was hoping to keep it in a non-volatile format.

User-burned DVDs do not last for ever. DVD drives in laptops are becoming more rare, and DVD drives in PCs will go the dame way. Remember diskette drives?
USB flash drives will eventually go the same way, but it will probably take a much longer time for them to disappear.
Windows 10 1809 is (over)due for replacement by 1903, so I would suspect that if you’re going to upgrade from 1803 in a couple of months’ time, then 1903 will be the then-current upgrade path. And it may even be more reliable that 1809 has proved (not) to be!

the updated 1809 ISOs MS posted when they declared it business ready near the end of March 2019 contain build 17763.379

indeed the revised x64 version ISO is close to 5Gb in size while the revised x86 version is less than 4Gb

Win10 ISO image too large for a regular DVD? Really?!?
Hmmmm. Well, then again, I guess that might make sense.

After all, a measly 4GB certainly would never be large enough to contain a compressed version of an operating system capable of running something as complicated as a computer!

And I suppose that’d have to be especially true for an OS like Win10, so Beneficially Laden with Original And Third-party Executables and DLLs (in a word, BLOATED)…

Mounting an ISO Image in Windows 8, 8.1 or 10
Double-click an ISO file to mount it. …
Right-click an ISO file and select the “Mount” option.
Select the file in File Explorer and and click the “Mount” button under the “Disk Image Tools” tab on the ribbon.
Then run the Setup.exe

I have what probably is a very naïve question: can one create a useful disk image in a USB pen drive? Some have a larger capacity than DVDs, these days.

The ISO is so large because it contains 7 different versions of Windows.

I like to get the .iso that includes both the x86 and x64 version for the DVD+R/DL disc and a 16GB USB drive as I never know ahead of time which I’ll be working with.  With the USB drive I can store a number of the succeeding Build updates as they become available.

The .iso that includes both the x86 and x64 version for the DVD+R/DL disc?
As for as I know you will have to Download TWO FILES ONE FOR 64 BIT & one for 32 bit version!
That is what I did on two 8gb USB drives.
Ron

I ran into the same problem a couple of months ago. The direct iso download from MS is indeed too large for a regular DVD (4.72 GB). However, if you download the iso via the Media Creation Tool it will fit a regular DVD because it is only 3.86 GB large. That is how I solved it. I do not have a direct link on hand but look for Media Creation Tool and then choose the iso download. That download includes both Home & Pro, but you have separate iso’s for x86 and x64.