pxelinux - boot image size for ms-dos boot

I am using pxelinux on a linux server to allow some diskless clients
to boot an MS-DOS boot image from LAN.

This works fine for 1.44MB images.

I also succeeded today in creating a 2.88MB (floppy image) with help
of
dd (grab the 1.44MB bootsector)
mtools (mformat a 2.88MB image file including the above bootsector)
copying io.sys, msdos.sys, command.com to the just created and mounted
image

But I need an even larger DOS-Partition via boot from LAN.
With my knowledges I reached the limit of 2.88MB by using mformat:

> mformat -t 80 -s 36 -h 2 -C -B fddbootsector x:
where x: is defined in ~/.mtoolsrc as the resulting image file.

I tried to increase the values in the mformat command, but the
resulting image was no longer bootable.

I tried to use an MBR of an existing HDD and the 8MB DOS-partition,
not bootable.

What am I doing wrong?
Any help possible?
At least, do I have the chance to go up to a 3.84MB floppy image? For
this I did not find any cylinder, sector, head numbers on the web. Do
you know the correct settings for such a floppy?

Thanks for any help.