Ark won't allow sizes greater than 1000 MB in the Multi-volume archive section of the Compress to Archive window.
Created attachment 110787 [details] compress multi 1200mb I can't confirm this on current Neon git stable (Plasma 5.12.1, KF 5.43.0, Qt 5.9.3, Ark 17.12.2). I was able to successfully make the multi-archive using 1200mb split option. Can you share some more details? Is there any error message or you just can't enter the bigger value (than 1000)?
file size limit in multi-part mode was increased some Ark releases ago.