If you have a 5 and a qtr inch bit to try on for size that would be your best bet before going out and buying the wrong size, but yes, a pinching bit and too handsy of riding will do that to a horse. Try putting some vetricin or something of the like on it. Keep them clean.
Can we see some pics so I can get a better idea of what they look like?
For the trail ride use a halter or a hackamore. Actually, use that until the sores are completely healed. It's like a cold sore, the moment you move it around it will just open again, so keep the bit off until there's no more sores.
Hopefully a tad larger size will help but overall softer hands will be the real solution so that sores don't develop in the future. Good luck!
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