For reasons that I forget... (IIRC has to do with how much space is allocated for the partition table on the physical disk) you can only make 4 entries in the disks partition table. Just as an MSDOS file system will only allow 512 entries in the "root" directory. To get around this restriction in MSDOS they came up with sub-directories, which were in the same format as the "root" directory with the exception that they could grow in size, as they were just files with a special attribute. Early versions of MSDOS did not have support for "sub-directories", IIRC that didn't come until MSDOS 1.6 or so.
So now we have "extended" and "LVM" types of partitions in the disks partition table to preform the same function as a sub-directory, it allows us to make more than 4 physical partitions. The physical size of the extended partition is determined by the number of logical partitions that you declare while running fdisk. It has the descriptions of the logical partition in it. A logical partition, cannot be made bootable, that is why you have to have a physical partition to boot from.
The output of sudo fdisk /dev/sda -l for my computer is below. I would recommend at least 10 Gib (some say 20) for the partition that you mount to "/". I for various reasons, like to have a separate partition for /usr (or with the 20 Gib option /usr/local) sized according to your experience and another for /home. The last one for a swap partition, sized (some say) to twice your installed RAM, I would make it so that RAM + Swap = 16 Gib for a "normal" desktop. I've had a machine running Apache, proftp as well as CAD programs and compilers running, with 12 G of RAM, run for months straight without using swap. Your mileage may vary....
Pardon the formatting of the output, this box doesn't seem to like my cut & paste!
Disk /dev/sda: 111.8 GiB, 120034123776 bytes, 234441648 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x3bec3746
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 11999231 11997184 5.7G 83 Linux
/dev/sda2 12001278 234440703 222439426 106.1G 5 Extended
/dev/sda5 12001280 111998975 99997696 47.7G 83 Linux
/dev/sda6 112001024 211998719 99997696 47.7G 83 Linux
/dev/sda7 212000768 234440703 22439936 10.7G 82 Linux swap / Solaris
Note: sda1 is the bootable partition (physical), while sda2 is an extended partion and it IS a physical, but just describes WHERE on the disk the "extended" information is located. sda5, sda6 & sda7 are logical disks described in /sda2. What happens to sda3&4? IDK, I used to, but that was back around kernel v0.97 or so, it's been awhile.
I hope this explains whats going on here.
For further info, I'd refer to an "old skool" book (copyright 1988 Thom Hogan) named "The Programmer's PC Sourcebook", it has a table of the Disk Partition Table, with all sorts of detailed notes on offsets, etc. If you want to get down to the nitty-gritty whatsgoingonhere stuff, it's valid today, except for the legacy OS information, well, the info is still valid, but who writes software to manipulate hardware on a IBM PC/2 anymore?
