unsigned int Mask;

void right_fill (unsigned char RightMostBits)
{
  unsigned char i=RightMostBits;
  unsigned int tmp=1;

  if( i )
  {
    do{
      Mask |= tmp;
      tmp <<= 1;
    }
    while(--i);
  }
}

is about 25 bytes with SDCC:

                             84 ;Allocation info for local variables in function 'right_fill'
                             85 ;------------------------------------------------------------
                             86 ;RightMostBits             Allocated to registers r2
                             87 ;i                         Allocated to registers r2
                             88 ;tmp                       Allocated to registers r3 r4
                             89 ;------------------------------------------------------------
                             90 ;rightmost.c:3: void right_fill (unsigned char RightMostBits)
                             91 ;       -----------------------------------------
                             92 ;        function right_fill
                             93 ;       -----------------------------------------
   0000                      94 _right_fill:
   0000 AA 82               104         mov     r2,dpl
                            105 ;rightmost.c:6: unsigned int tmp=1;
   0002 7B 01               107         mov     r3,#0x01
   0004 7C 00               108         mov     r4,#0x00
                            109 ;rightmost.c:8: if( i )
   0006 EA                  111         mov     a,r2
   0007 60 0F               113         jz      00106$
                            114 ;rightmost.c:10: do{
   0009                     116 00101$:
                            117 ;rightmost.c:11: Mask |= tmp;
   0009 EB                  119         mov     a,r3
   000A 42*00               120         orl     _Mask,a
   000C EC                  121         mov     a,r4
   000D 42*01               122         orl     (_Mask + 1),a
                            123 ;rightmost.c:12: tmp <<= 1;
   000F EC                  127         mov     a,r4
   0010 CB                  128         xch     a,r3
   0011 25 E0               129         add     a,acc
   0013 CB                  130         xch     a,r3
   0014 33                  131         rlc     a
   0015 FC                  132         mov     r4,a
                            133 ;rightmost.c:14: while(--i);
   0016 DA F1               135         djnz    r2,00101$
   0018                     136 00112$:
   0018                     137 00106$:
   0018 22                  138         ret

(25 Bytes are including RET and MOV R2,DPL)