When selecting the partition to use manually, you can enter the size of the partition. So my question is, if I want 8GB swap size for my 8GB ram (yes I know the FAQs about the size of the swap etc) , am I supposed to select 8000MB as in 1GB = 1000MB or 8192 as in 1GB = 1024MB?
TL;DR: It does not really matter.
RAM is universally measured in powers of 2. This is often written as GiB, so 8GiB is 8*1024=8192MiB RAM.
If you look at physical RAM chips they'll always come in powers of two, e.g. 1024MiB, 2048, 16384MiB etc.
Hard drives are typically measured in powers of ten, using SI prefixes, for instance, 80GB = 80*1000MB.
So 8GiB of RAM is 8192MiB of RAM. For swap, you typically want a swap space that is big enough to hold all of RAM for suspension, so 8192MiB would be the safe option. Note that it really doesn't matter, as Linux will almost never use 100% of RAM anyway; a lot will be used for caching and so on, which will not be saved when you hibernate.
It can matter very much, which size the swap-drive is. My swap-drive is a partition shared by all ten (10) of my multi-booting Linux operating systems. If the swap-drive is on the motherboard's spinning HDD, the sector-cluster is fixed by the physical sizes on the spinning disk. If the swap-drive is on a flash-drive or SSD, the size is "electronic" rather than physical, so it matters much less if the size is exactly of factor of 512 bits.
Using "gparted" or similar, allows seeing the effects of choosing the wrong partition size. A strange unused partition appears if the wrong numbers are chosen. Often changing his to upsize or down-size will make these strange left-over partitions disappear.
In Linux with 16 GB of DDR3 memory on m Dell notebook, the swap partition is never used by the Linux operating systems. However, when running "live" Linux operating systems, it may be used or needed.
Swap partitions in Linux behave differently in Microsoft Windows. My Dell notebook has also three (3) Windows-10 operating systems as well. Even though the setting is zero swap partitions, Windows-10 always will demand a swap partition on the booting partition. So I create a tiny, fixed size partition there. The on-board SSD has a variable swap-file on it, to be shared by all Windows-10 operating systems.