Section 12.6.2 in ACI 318 stipulates lap splice length with bars of different size when bars are in compression.
Is there any such guideline for bars in tension?
Seems to be new in 318-08, though it isn't noted as such.
If you don't have 08 then...
"12.15.3 - When bars of different sizes are lap spliced in tension, splice length shall be the larger of ld of larger bar and tension lap splice length of smaller bar."