to enlarge a block, on each of the pieces you must first eliminate the seam allowance, then enlarge a consistent percentage on each piece. So, if you want a 4" finished block enlarged to 4-1/2", that is an increase of 1/8 or .125. (Multiply 4 x 1.125). Then add back the seam allowances to each piece.
Not sure if this is exactly what you're asking though...If you're looking to make the block slightly larger, you could decrease your seam allowance a tad. Or when you cut the pieces, use the outside of the line on your ruler instead of centering the line exactly on the edge of the fabric. This should work if you cut all the pieces using the same side of the ruler line and might add enough to make your block slightly larger.
Laurie