[Cherish]

I don't really think that there isan ideal size for a guinea pig.

You get different sized pigs, just like you get different sized people, and there are a lot of factors that contribute to the size of your guinea pig.

For example, I have a five month old boy who is huge,but that's not because he's overweight, it's beause he's a Rex, and Rexes tend to grow much larger than most other breeds.

So don't worry about your pigs being an "ideal" weight, just let them grow to a weight that is ideal for them.
Keep a check on them to make sure they don't lose weight, and if you're feeding them correctly, that's all you need to worry about.