Hello,
splitting by file size (a new file whenever a maximum size is reached) is not possible yet. It is on our to-do list.
Until then you can split by number of pages: in the PDF24 Toolbox, open Split PDF and choose the mode Pages per PDF. Your file has about 0.2 MB per page on average, so 12 MB are roughly 60 pages. As pages differ in size, start with a lower number, for example 40, and check the sizes of the new files. If a part is still too large, you can reduce it with Compress PDF.
For a 1 GB file, the PDF24 Toolbox on your computer is the better choice: https://tools.pdf24.org/en/creator
Splitting also works online: https://tools.pdf24.org/en/split-pdf