Metric: 16cm x 13cm w/ 45° off each corner (13mm).
Inches: 6-1/4“ x 5-1/8” w/ 45° off each corner (1/2“).
3/32” / 0.0925“ / 2.35mm
Doesn't have to be this exact thickness.
4 beds - $3.38 ($0.85 each)
11” x 14“ x 3/32”
1 bed - $1.98 ($1.98 each)
8” x 10“ x 3/32”
Note about Home Depot online listings:
Listings say thickness is 1/8“ or 0.125”, Reviews and personal purchase of in store product confirms it is 3/32“
A “Z-spacer” may be needed but not always.
See the dedicated Z Spacer Thingiverse collection page or take a look at few listed below.
Monoprice Select Mini Glass Bed Spacer
MP Select Mini Z Spacer (customizable, no disassembly!)
MP Select Mini Z Spacer (snappable)
There are many ways to attach the glass to the bed to prevent it from moving while printing. See the Thingiverse collection below or on the dedicated Glass Bed Clips Thingiverse collection page.
Another option is Silicone Thermal Pad.
400mm x 205mm x 0.5mm Silicone Thermal Pad for CPU GPU Heatsink
Adding A Glass Bed - 3dprinterwiki.info
This is a pretty good guide on how to add a glass bed using silicone thermal pads. Is is meant for the Wanhao i3 and clones but should give you the idea.