I own these in brown
I picked these up a week ago and used some last night. They are thick enough but also have a bit of bend to them. I had to apply a bit of weight to be sure that they completely adhered to the paper. Happily, since then they have definitely stayed put so they definitely stick!
I plan on picking up more of these packs in the future. the letters are about 2 inches long and 1 1/2 inches wide. Most consonants have two of each and most vowels have three -- that's always something I look for when stocking up on alphabet pieces. I thought it might help.