What is the max SD card size I can use in my DSi with HiyaCFW and DSiMenu++ ?

You would need to format the 64gb+ SD to SDHC, not SDXC. Doing that does slow down the SD, so you really shouldn't try it.

 

There's indeed no such thing as "formatting to SDHC"; however, the standard for regular SDs mandate them to have a MBR partition table containing only one FAT16 primary partition, the standard for SDHCs mandate them to have a MBR partition table containing only one FAT32 primary partition, and the standard for SDXCs mandate them to have a MBR partition table containing only one ExFAT(FAT64) primary partition

There's absolutely no technical reason to tie a generation of SD to a specific partition table or filesystem (lobbying of members of the SD association, especially towards the still-patented ExFAT, is probably the main reason); however that's how the standards are written and the only reason why many devices supporting SDXC cards (which have no significant differences with SDHC) but not ExFAT can't advertise "SDXC support"

 

Short answer is: Yes!

I tested an SDXH with 128GB successfully on a DSi and don't see a reason why bigger capacity would not work. But I don't see any reason to actually use such big SDs on a DSi either.
Please note that (modern) Windows does not allow formatting partitions >32GB to FAT32. If you are WIndows user guiformat will do.

 

Linux: mkfs.vfat
Windows: guiformat
I don't have a Mac but I guess Mac OS has built-in FAT32 formatting tools (hopefully not with an artificial limit like Windows)
Mostly recommended cluster size is 32KB.

Anything ≤ 2TB can be easily formatted to FAT32

Unless you manually changed the partition table on your SD card to GPT there is nothing more to do than reformatting.

 

I tried many sd-cards >2GB ... they do only work, when sd is filled with data <2GB. More than 2GB causes an black screen error an twilight++ is not booting.
2GB can only formatted with 16KB clusters, not 32kb. Its working. When using more partitions on SD, install in primary partition.

 

TWiLight Menu++ can work perfectly fine on large cards, its just HiyaCFW that doesn't work well. So if you're using TWiLight Menu on a large card I'd recommend switching to launching it directly in Unlaunch instead of through HiyaCFW, unless you do want to have the HiyaCFW SDNAND menu too in which you will need to fill it up.