Hello, I have set up dhcp option 108 ( RFC 8925 ) in a box with 7.15rc2 with
/ip dhcp-server option
add code=108 force=yes name=nodhcp value=“‘1800’”
The RFC says
“The server MUST set the length field to 4. The client MUST ignore the IPv6-Only Preferred option if the length field value is not 4”
but my box is sending the length as 2 and my hosts is ignoring it then.
So there is a bug or a feature missing where I can set the length.
That’s because the /ip dhcp-server option feature doesn’t care how the individual options are specified in whatever RFC. It’s not its job to know that for option X the value should be formatted as Y and has a length of Z. It gives you the ability to the enter the raw content of the option, with some help for conversion.
If you enter 1800, it sees a decimal number and convert to a number of bytes big enough to represent that number, in this case 1800 can be stored as a 16bit bit number, so 2 bytes is used. If you want exactly 4 bytes, use the 0x1234abcd format. In this case 1800 translates to 0x00000708
Here some value ref for how to code the correct options with correct syntax.
Look for value, and you can see how the line option being decoded.
And if you are knowing the hole raw byte stream you could also check what the value being encoded to in the raw options in hex of course.
And the best thing to debug this is via wireshark.
https://help.mikrotik.com/docs/display/ROS/DHCP#DHCP-DHCPOptions.1
If I set it to
set value=“‘9999999999’”
The lenght is 4 and it works! 
Or you can enter 0x00000708 like I wrote above and have your 1800 value as 4 bytes.