One half-size M.2 PCIe/SATA drive is it 2260 or 2242 for factor of the ssd

There is no such "half Size" specification.   There is 42mm and 60mm and 80mm and 120mm. MSATA is LARGER THAN ANY M2.  MSATA IS ON THE LEFT and 42mm M2 is on the Right.  You are asking about that which DOES NOT EXIST.

This socket for example supports 2260mm and 2280mm  they are all the same width the Length is the only thing that changes.

There are conversion kits.  This one holds 30mm, 42mm,60mm, 80mm

Hi ‌,

What's your system model and revision?

I own a laptop Alienware 17R4. And its writen that it suppurt 2 full size m.2 ssd and 1 half size. And i am little bit in doubt what form factor is for this half size m.2 ssd (ssd1).

The following table shows the hard-drive options available in this computer.

So, the computer has 3 ports M.2 PCIe/SATA:
Two in size 2280, this one runs at 4x and one in size 2242 that runs at 2x.
If you place a 4x in the 2x slot, it will work at 2x.

You're welcome

Thanks a lot.

http://topics-cdn.dell.com/pdf/alienware-17-laptop_Service%20Manual_en-us.pdf 

Doesnt look like #2 is user applicable. The one on the bottom in the manual is clearly 2280.

The Bios Page 124 is also confusing as it talks about 3 M2 drives.  There is no mention of install or removal of SSD#2 and no mention of #3 in the service manual.

No idea what holds ssd #2 onto the motherboard.