The Universe today is about 14 billion years old. If the Universe were not expanding, this would mean that the width of the Universe we could see would be about 28 light years across.
However, since the Universe is expanding, the width of the Universe we can actually see is about 100 billion light years across. This is discussed in this space.com article.
How big was this earlier in the history of the Universe?
The Cosmic Microwave Background was emitted when the Universe was about 400000 years old. When this happened, the size of what is visible now was about 1000 times smaller than it is now, or about 100 million light years across.
At the end of inflation, the Universe was probably about 1027 times smaller than it is today. So, at this point, the width of the Universe we see today would have been about 10-16 light years, or about a meter across.
In order to make the Universe to look flat, homogeneous and isotropic, the Universe must have expanded by a factor of at least around 1026 during inflation. Combining this with the size just after inflation (directly above), at the beginning of inflation, the Universe would have been about 10-26 meters across. I don't know of anyone who uses these units, but this would be one one-hundredth of a yocto-meter. To be a bit more concrete about it, this would be about 100 billion times smaller than the classical size of a single proton.