I’m looking for micro sd card for pi.
I can’t find this in specs enywhare.
How fast can Pi4 write to sd card?
P.s
Bonus question.
What was the biggest capacity card you used with Pi4 ?
I advice you to go with a cheap 128 GB SSD, they are so cheap and give a big performance boost. The pi 4b supports USB boot now.
Here is a link with a lot of SSD/SDCard benchmarks with the Raspberry Pi…
According to Raspberry Pi’s official formatting instructions, Raspberry Pi’s bootloader only has support for reading from FAT16 or FAT32 filesystems. User more than 32GB SD card requires you to reformat the card to FAT32 before doing anything else 
,speed refers to the SD class etc. Another option may be what you’re talking about @chomsky
And that should be more then enough. I have one usb ssd and I use it as a pendrive
That depends on your sd-card.
You can use just about any sd-card size as the bootloader is on a small partition - and the system on the rest - using ext4.