@Isabel-Reyes-Feeney while it's good not to over-stress and overthink, it's also good to "begin as you mean to go on", as the saying goes. Knowing standard bleed, margins, gutter, and how to leave space for text is essential for an illustrator. So yes, it's good if your portfolio demonstrates that you know how to do that.
Trim sizes can vary widely, but most picture books fall somewhere between 8-10 inches wide x 8-11 inches high. It's rare to see a picture book taller than 12 inches high, since the clearance on most shelves is 12 inches. And librarians and parents hate it when books don't fit on shelves!
Standard bleed is 3mm, or 0.125 inches.
Margins vary, but the bare minimum for margins is usually 6mm, or .25 inches. However, when designing a book, I like to keep margins around 1 inch. That's because the book is going to be held by big and little hands, so I try to avoid the possibility of thumbs obscuring the text.
For the gutter, if you have a safe area of about 12mm, or 0.5 inches, you should avoid any essential elements of the illustration getting lost in the gutter. *However, if you're leaving space for text close to the gutter, you need a larger safe area. I try to keep any text 1 inch away from the gutter on either side, so the gutter safe area for text would be around 48mm, or 2 inches.
For a visual of this, here is a template that I made and use for all of my illustrations. The red area represents the bleed. The blue areas are "unsafe". The blue lines divide each page of the spread into thirds and help me with composition. Feel free to use this template. That way you don't have to measure things out every single time, which is what I used to do and it can get tedious. It's currently sized for an 8x10 inch trim size, but it's easy to adjust by changing the canvas size in Photoshop/Procreate/program of your choice.
6657be38-e058-40ce-bd73-768f76bc2239-spread template - 8x10 layout.jpg
Hope this helps! đ