Is a 0.31 carat diamond big enough for an engagement ring?

It is on the smaller side but common for everyday rings. On an average finger it reads as a delicate solitaire rather than a statement stone.

How does this compare with a halo setting?

A halo adds small stones around the center and makes the diamond look larger. This plain solitaire gives a cleaner, more classic look with less total sparkle.

Is platinum a good choice for a daily-wear ring?

Yes. Platinum is dense and durable and develops a soft patina over time. It costs more than white gold and is heavier on the hand.

Can the ring be resized?

A plain platinum solitaire band can usually be resized by a jeweler. Have it done by someone experienced with platinum, since the metal needs different handling than gold.

What should I check on the diamond?

Ask for the cut, color, clarity and certification details for the specific stone. Cut has the biggest effect on how much a diamond of this size sparkles.