First, the bathroom floor...if you want hex tiles, the only reason you might not be able to use them is if you are planning on using a membrane like Ditra under the tile. Ditra, due to the nature of it's structure, limits tile to a minimum of 2".

If you're going over cement board or another smooth-surface tile backer board on your floor, you can use small hex tiles.

Simply ask him what his no kidding objections are to the hex tiles. There may be a real world reason why they are too small (like over Ditra) or he simply may not like working with mosaics. Find out why.

For the shower floor, I usually stick to 4" or smaller, that way there are grout lines underfoot for traction. Plus smaller tile conform to the slopes of the pan without requiring cut lines at the intersections of the slopes in the pan. 2" squares are probably most common. Some people like the flat stone or pebble mosaic, but if you suffer from grout-phobia, the irregular spacing on those might scare you away.

If there is a trend these days, it's toward large format tiles. They can lead you towards a contemporary design, but they can fit other styles as well.

If I can steer you towards anything, look at "topical waterproofing membranes" for waterproofing your shower. Topical membranes put the waterproofing between the tile and the tile backer board, so the tile get adhered right to the membrane. It's a better way to control moisture within a shower. Two of the more common trowel or roll-on membranes are Hydroban and RedGard, for sheet membranes, Kerdi is probably the most common one.

Probably the best, and easiest way to waterproof a shower these days is with a Laticrete flanged drain and Hydroban. You can wade through youtube videos or visit Laticrete's website to see what that is all about.

Remember...unless you are planning on selling, I'll caution you to use tile sizes and patterns that you like, that meet the style of your house. Don't design for the latest fad, unless it's something that you really prefer.

If you have trouble defining the tile size, pattern, or style that you do like, it can sometimes help to go to a site like houzz and look over bathroom photos there.

Bathrooms should last 20 years or more, so what's currently popular isn't really something to design around. What's really popular today will look very dated in a year or two. The small octagons are a classic.

But back to the original question: Larger tiles are becoming more popular. I happened to be at the tile store today and in many lines the smallest tile is 12x12, with 18x18 and 18x36 being very easy to find.