Very large bubbles in starter

Toast

So I have been trying to cultivate some sourdough starter (1:1 flour water ratio) and feeding it twice a day. I always get large bubbles (as opposed to lots of tiny foamy ones) in it that pop when some is scooped out and thus I cannot do the check of it floating in water to see if it is ready to use. Is it OK that my starter is like this? Do I need to use more water? Why are the bubbles so big and easy to deflate?

Honestly, I don't go by the float test ever since 1) the gas deflates as I'm spooning out the starter and 2) just use the starter when it's doubled in size or (since sometimes young starters don't double in size) when it looks...well, very bubbly.  I've also used my starter a bunch of times when it wasn't fully active and it still produced great loaves. If you look at your starter and it seems like there are a bunch of big and small bubbles running throughout, then it's probably good to go. Looks like it's close to that stage in the picture you posted! good luck

I am a new bread baker, so that k you for the advice! My starter has been going for a week now, and is starting to rise and fall regularly; how do I know when a starter is ready to be used in bread and left in the fridge and stuff? How long should I let the yeast cultures mature before trying to bake with them?

Yup, keep doing the same. When it rises and falls consistently every 5 hrs or so (or less), try making some bread (try a simple recipe and a small loaf so you're not wasting too much). It's looking good!