I've got a little truck beater build I threw together
DSarms 14.5" carbine gas M4 barrel
DPMS C7 upper with M4 ramps
Toolcraft BCG
Palmetto Lower with carbine buffer and spring

One thing I noticed before taking her out is that the plastic carbine handguards we're almost impossible to put on with just one person.  It made me curious if maybe the FSB was a c-hair too far back from the muzzle.

Me and my son took her out with a handful of Tula steel ammo to sight it in.  We were able to get it zeroed but it could barely cycle the ammo.  We tried putting one round in the magazine to see if we could get the bolt to lock back.  If failed almost every time.
I'm going to pull the FSB and check the gas port.

Any suggestions on what gas port size should be or what size drill bit should be used if I need to open her up a little would be appreciated.

I pulled the barrel.  I pulled the FSB.  My plastic harbor freight calipers were useless to measure the port.  We picked up a #51 (.067") drill bit.  We put a dowel down the bore and carefully drilled out the gas port with a cordless drill.  
We ran a brush down the bore a few times to clean it out.
Reinstalled everything.  Got her juicy and brought a magazine full of Tula.  She dumped the mag and it locked back after the last round.  
So far so good.
I have to credit this archived post.  It's exactly what I did.
Not sure who it was but Thank You.

"A mil-spec 0.063" gas port will only ever be a problem if you're shooting extremely weak ammo.  In that case, a slightly larger gas port "may" be desirable.  That said, something on the order of 0.067" would be sufficient.  That's what Noveske used when they made carbine length N4 light carbine barrels."

The problem was likely the bolt carrier group.  A .0625" port is milspec and will push an H2 buffer without issue.

If you needed to open the port beyond that with a carbine buffer, you have some massive gas leaks somewhere to address, it sounds like.

I wish I would have gotten the proper tools so I could actually have checked what the gas port diameter was before drilling so that I could have been certain.

Gas port data from a guy on the internet

I can't attest to the accuracy

Tula ammo is known to be under-powered.  You should have used a different and better ammo first and before taking things apart.

Yes.  Dial calipers might not be the right device to measure a small hole.  You are correct.

0.063" for 5/8" diameter
0.070" for 3/4" diameter

use wire gauge drill bits (numbered) to measure
#53  0.0595"
#52  0.0635"
#51  0.067"
#50  0.070"
#49  0.073"

Recently just picked up a set of wire gauge drill bits.  probably should have done this from the get go.