You've gotten some great ideas here already!

We've done smaller binders, separated out for history, science, and copywork before, and enjoyed that too. I do think it's a good idea to consider the space you have and do what works best (a.k.a. what won't drive you nuts all year

)! I think the sketchbook could work for the copywork of the poetry very nicely. You could have a 2 page spread with the poem copied on one side, and the child could illustrate the poem on the other side. (One of my sons loved to illustrate his poetry, the other one not so much. So you can play that by ear.)

As far as the science, if you look on Day 3 in the Science Exploration box of the daily plans, you'll see the steps for doing the lab sheets, and you can make a standard one if you'd like to lessen the writing for the child. PHFHG has a standard lab sheet for that can be copied at that point, but in BHFHG, the idea is to have the students write and remember the parts of the experiment to remember them better. However, for my young 7 yo doing Bigger, I did make a lab sheet for him, as the other writing in Bigger was/is enough of a challenge.

A sketchbook would be neat for the science experiments too, otherwise.

Here's a link for my getting ready for Bigger, but of course, my way of doing it is just one way, so be sure to customize it to fit your needs:
viewtopic.php?f=6&t=5531
The "Introduction" of Bigger Hearts is important to read as well, as it gives options for storing dc's work and can help as you choose what's best for your family. HTH!
In Christ,
Julie