Size requirement for KVM installation

Could you please provide min space requirements for KVM installation?

I’ve read that the sparse file can actually cause problems when ran on smaller drives.
Is 256GB flash drive enough for lightweight Debian Buster installation (max 10GB) and then running KVM Whonix on it? Real capacity of 256GB drives is 238GB. Is it enough?

Yes, it is more than enough. Sparse file means it will grow over time when for instance you download stuff or upgrade the machine, etc. You can recompress it later using tools like zerofree and qemu-img.