Choose among three footprints of same chip

[Wilson__]

From factory web, there are three footprints.  Presumably, different footprint fitting different process? 

Which one should I use?  For single quantity prototype, hot plate, hand place under microscope. Tin/Lead 63% to 37%, (may be activated) resin flux, wash with isopropyl alcohol.

All pads have same central line agreeing with data sheet.  0.35mm between two small legs.  0.65mm between small and large legs.

a) plain   small leg 0.30 x 0.20mm, large leg 0.30 x 0.56mm
b) L        small leg 0.20 x 0.15mm, large leg 0.20 x 0.50mm
c) M       small leg 0.40 x 0.25mm, large leg 0.40 x 0.60mm
(was swapped number small leg 0.40 x 0.25mm, large leg 0.60 x 0.40mm)

EDIT: 
data sheet  small leg 0.25 x 0.15mm, large leg 0.25 x 0.50mm
Plain is middle-of-road.  M is bigger.  L is smaller
The chip is lead-less (SON/QFN).  Solder pads under chip.  No wetable side wall.

many thanks

[PlainName]

I'd probably go for A-plain. Reason: larger pads to make placement easier manually, but small enough to locate the part properly when the solder melts and not have bridges.

[Wilson__]

L is equal or slightly smaller than chip pad. 

Wonder if L is for thicker stencil.  Solder paste has smaller footprint and taller, for same volume of solder paste.

[SMTech]

L is equal or slightly smaller than chip pad. 

Wonder if L is for thicker stencil.  Solder paste has smaller footprint and taller, for same volume of solder paste.

None of them. You are building single (not an ideal thing in itself) prototypes with next to no equipment, you need to know your assembly is good so you can focus on your design and validation. Pick something workable you can inspect place and handle easily.

This device and devices like it present a whole heap of challenges you are not equipped to deal with. Two of these packages are firmly in microBGA scale territory, these often imply a multi-level stencil to handle the mix of power and fine pitch components found on the board.

There are a heap of reasons seemingly similar footprint might exist for one device, you haven't given the package sizes so we can mostly just guess.
1)The overall size is different enough it matters in circuits where being tiny matters (e.g a phone/watch)
2)The device can be used as a replacement part for a rival or discontinued part that was in one of those other packages
3)They wanted one option that needed  less capable equipment to place.

There is a ratio for the width/area of the pad vs the stencil thickness, step outside it and the paste stays in the stencil aperture.

[Wilson__]

L is equal or slightly smaller than chip pad. 

Wonder if L is for thicker stencil.  Solder paste has smaller footprint and taller, for same volume of solder paste.

None of them. You are building single (not an ideal thing in itself) prototypes with next to no equipment, you need to know your assembly is good so you can focus on your design and validation. Pick something workable you can inspect place and handle easily.

This device and devices like it present a whole heap of challenges you are not equipped to deal with. Two of these packages are firmly in microBGA scale territory, these often imply a multi-level stencil to handle the mix of power and fine pitch components found on the board.

There are a heap of reasons seemingly similar footprint might exist for one device, you haven't given the package sizes so we can mostly just guess.
1)The overall size is different enough it matters in circuits where being tiny matters (e.g a phone/watch)
2)The device can be used as a replacement part for a rival or discontinued part that was in one of those other packages
3)They wanted one option that needed  less capable equipment to place.

There is a ratio for the width/area of the pad vs the stencil thickness, step outside it and the paste stays in the stencil aperture.

Many thanks for advises.  May be I can try.  If fail, can pay professional to do it.

Wonder if that chip is comparable to imperial 0201. 

Since the large pad is much bigger than the two small pads, reflow surface tension may move the chip mainly by effect of the larger pad.

Land size below is comparing with 0201

--------
chip package size is 1.0 by 0.6mm

For plain,   land of large pad is 0.3 x 0.56mm,  area is 0.168 mm2
For 0201R, land is 0.4 by 0.45mm,                  area is 0.180 mm2

[NorthGuy]

This is more like 0402, which is 1 mm x 0.5 mm. Although 0402 is much easier to place manually - if you misalign 0402, it will align itself. If you put your chip at a slight angle, wrong pads may get soldered.

The size of the holes in the stencil determines the amount of paste. IMHO this is more important than the size of the pads. Also consider the gap between pads. If pads are too close together and you put too much paste, they may bridge. Perhaps spreading two small pads a little bit further apart than 0.35 mm may be beneficial.