Looking for Raised Garden Bed Advice

I'm building some raised garden beds for my mother-in-law. We're planning on, quite simply, using planter wall-blocks and 2X6 planks from Home Depot.

In the past, when I've built raised Garden Beds, to keep out moles and such we've done a layer of hardware cloth, followed by a layer of barrier fabric to keep grass from growing up through. Then we've just had the soil on top of that. My mother-in-law, however, wants to do a layer of tarp on the bottom, followed by a layer of gravel on top of that. She explained it's supposed to help with water retention or something like that? Thoughts on gravel / tarp in a raised garden bed? My fear is the tarp would negatively impact water drainage.

I was going to look up the cost of materials and get her the cost per bed so she can determine how many beds she wants. When I asked what dimensions she wanted, her response was "I am flexible." Obviously, the size depends on what you want to plant. Are there standard raised garden bed dimensions? I mean, if we're doing gravel on the bottom, I'd want to do at least 18" walls, but is there reason to do higher than that, and would there be standard length x width dimensions I ought to quote her?