so..as / as..as

1a. The pencil was so long as the paper.
1b. The pencil was as long as the paper.
2a. I’ll go, so long as it doesn’t rain.
2b. I’ll go, as long as it doesn’t rain.
Please correct the above.
Thanks.

1a is incorrect. The others are all in regular use.

Beeesneees,
1a. I’ll stay so long as you need me.
1b. I’ll stay as long as you need me.
2a. So long as you offered, I’ll take another slice of pizza.
2b. As long as you offered, I’ll take another slice of pizza.
Please correct the above.
Thanks.

When we wanted to make comparisons in the past, we used the expressions ‘so … as’ and ‘as … as’ with the negative and the positive sentences respectively. Modern grammar, however, approved of the use of the latter in both the cases.
(I’m talking about the very first two sentences posted)

1a/b. I’ll stay as long as you need me.

2a/b. As long as you are offering, I’ll take another slice of pizza.