[dataweaver]

OK: using the following as the baseline,

Size  Str Agl
  1   -2  10
  2    0   9
  3    2   8
  4    4   7
  5    6   6
  6    7   4
  7    8   2
  8    9   0
  9   10  -2

…and assuming that Corporeal critters tend to have a total number of Forces equal to their Size, here's how Str and Agl are likely to pan out:

Size Forces Str Agl (Forces is "Corp,Eth,Cel")
  1   1,0,0  0  12
  2   1,1,0  2  11
  3   2,1,0  6  12
  4   2,1,1  8  11
  5   3,1,1 12  12
  6   4,1,1 15  12
  7   5,1,1 18  12
  8   6,1,1 21  12
  9   7,1,1 24  12

Note that I'm assuming that the typical Size 5 animal will have more Corporeal Forces than the typical human; that's because humans put more emphasis on the Ethereal than animals do.

Note also how Agility holds pretty much steady around 11 or 12; that's by design.

The only assumptions I'm making about characteristic points is that you can't "sell down" a characteristic below its base level, and you can't have negative scores. There might also be a ceiling imposed (e.g., at Size 5 you can't put more than 10 characteristic points into each of Str and Agl, with lower caps for smaller sizes and higher caps for larger sizes); but if so, steps up from human size should add at least two to each cap: otherwise, you'll run into cases where you won't be allowed to spend all of your characteristic points. Assuming that you're not allowed to allocate more than 2xSize to any one characteristics, here's the best you can possibly do:

Size Forces Str Agl (Forces is "Corp,Eth,Cel")
  1   1,0,0  0  12
  2   2,0,0  4  13
  3   3,0,0  8  14
  4   4,0,0 12  15
  5   5,0,0 16  16
  6   6,0,0 19  16
  7   7,0,0 22  16
  8   8,0,0 25  16
  9   9,0,0 28  16

Conversely, here's the worst you can do:

Size Forces Str Agl (Forces is "Corp,Eth,Cel")
  1   1,0,0  0  12
  2   0,1,1  0   9
  3   0,2,1  2   8
  4   0,2,2  4   7
  5   0,3,2  6   6
  6   0,3,3  7   4
  7   0,4,3  8   2
  8   0,4,4  9   0
  9   1,4,4 12*  0*     (* or 11 and 1, or 10 and 2.)

Note that Sizes 1 and 9 require at least one Corporeal Force each, in order to offset the negative Strength and Agility respectively. At the low end, you have no room to maneuver: you must spend 2 characteristic points in each. There is not going to be a Size 0. At the upper end, you have a little room to maneuver. If we were to extrapolate the chart to larger sizes (not recommended), you would need to devote another Corporeal Force every other step to offset the ever-increasing Agility penalty. Note also that in practice, you're not going to be hitting these minimums above Size 3 or so; and even that's pushing it: animals simply won't have anywhere else to put their Forces.

A slightly simpler approach: stop at Size 8; anything larger than that is the physical equivalent of a Superior. And when "downsizing", don't let baseline Strength go below 0. I'd say that elephants would be Size 7, and you'd have to go to extinct animals or the oceans (e.g., dinosaurs and some whales) to find Size 8 critters. So:

Size  Str Agl
  1    0  10
  2    0   9
  3    2   8
  4    4   7
  5    6   6
  6    7   4
  7    8   2
  8    9   0