1200mm or 4 feet would be ideal.

How about butcher block instead? IKEA and the big box stores sell it for countertops, the price is decent, and you won't have as many issues with buckling, water damage, or what to do with the edge as you would with the ply. If the edges of the ply get wet, or if any seam around a sink gets wet, the plywood will "blow" and you will have to replace it all, anyway.

Lighting - pendants over the servery bench should be another feature that complements your chosen kitchen surfaces. So whilst I love the copper pendants in picture 1.. They could work depending on the rest of your decor and would be a highlight in a neutral white/timber kitchen = Good! If you choose copper you would complement it with a couple of copper bench top items. These are beautiful! (I collect copper because I love it, my kitchen pendants are silver, I renovated to sell in the next 3-5 years) You will see a picture starting to form in your mind and there is nothing wrong with making a statement with your lighting. I found that when I wasn't sure I would get a very strong sense of what was right and what was wrong for a space, then common sense or passion would determine the outcome! Here are another couple that might work, look her on Houzz under lighting, search pendants, then have fun looking!Timber tones with black or white to complement your cabinets... or Statement white pendants for a bit of quirky fun! Your personality can be reflected in your lighting and accessories creating that point of difference from the predictable white on timber look. Plus don't forget the power of greenery to punctuate the theme!

If you mean a seam? Maximum length they can make a piece without a seam is determined by the length of the slab available.