*, a NT and psalms could work. The size is the main thing.
In that case, why not one of those Gideons NT/Psalms/Proverbs editions? I have a KJV and a NKJV, and they are small and free (I got mine free, anyway). I keep one on me just in case I want to read a bit during a free moment or if I have to pray with someone unexpectedly, so I don't use it to pray the Psalter regularly, but it could work for you.
Also, there is an Orthodox NT/Psalms
of similar size which was intended as a "military" edition, but you can buy it cheap. It uses the RSV, I think, and has some basic prayers and other material in the back. It is nice.