To change the layer length in the BPix you want to change the number of modules per half rod in the “Barrel PXB” section of PixelTechnicalProposal2014.cfg:

Barrel PXB {
  numLayers 4
  numModules 4   <---------------- this what you want to change
  innerRadius 29
  outerRadius 161
  width 22   <------------ this is the module length along x (local x)
  length 71 <-------------- this is the module length along z (local y)
  
  .....
  
  }
  

One module is always located centrally so that the number of modules in a rod is always an odd number. For example, numModules 3 will result in 7 modules per rod.

You can actually change the length of a layer (including innermost and outermost layers) by specifying the number of modules to use/rod for this layer:

Barrel PXB {
  numLayers 4
  numModules 4   
  innerRadius 29
  outerRadius 161
  width 22   
  length 71     
  ...
  
Layer 1 {
    radiusMode fixed
    placeRadiusHint 29
    numModules 3
  }
  
  ...
  }

Here are some resolutions after making the following changes:

• extend layer 1 and 2 by one module
• bring layer 2 closer to beam pipe (58 mm)
• move layer 3 and 4 out (120 mm, 180 mm)
• replace disc 1 (of 2 rings) with 3 discs of 1 ring each (at 265 mm, 285 mm, and 335 mm)

1 GeV	layout 02						TDR				
	C	I	F	VF	WF			C	I	F	VF	WF
pT	1.84731	2.88889	3.91328	3.84539	11769598		2.34579	3.62749	3.65583	3.90297	11769598
d	0.00608	0.0077	0.01112	0.0151	39535			0.00625	0.00816	0.0112	0.01446	39535
phi	0.00209	0.00292	0.00412	0.00534	10850			0.00224	0.00321	0.00415	0.00482	10850
z0	0.00748	0.01362	0.03911	0.11751	0.25247		        0.00723	0.01342	0.03911	0.11134	0.24704

IDEAL
1 GeV	layout 02						TDR				
	C	I	F	VF	WF			C	I	F	VF	WF
pT	0.49249	0.64322	0.70456	0.88607	523298			0.55644	0.86291	0.68587	1.01757	523298
d	0.00242	0.00245	0.0025	0.00334	1557			0.00255	0.00268	0.00248	0.00406	1557
phi	0.0006	0.00069	0.00068	0.00086	454			0.00062	0.0008	0.00066	0.00102	454
z0	0.00256	0.00262	0.00355	0.01215	0.06125			0.00277	0.00288	0.00358	0.01828	0.06125