When you eFile, the only information needed is caliber and length.
As for how big the bore can be:
Rule of thumb is 0.060 over the caliber to prevent baffle strikes.. For a .30, that would be .360 or a 23/64 (.3594") drill bit. Some people just go with the 3/8 (.375) and call it good.
A 9mm is ~.355". If everything is perfect and nothing ever went wrong, you certainly could fit that through your .360 or .375 hole. But I'd bet dollars to doughnuts you'd end up with a pretty devastating baffle strike. To get enough clearance, you'd need either a "Z" (.4130) or a 27/64 (.4219) bit.
The honest truth is, if you want a can that'll do 9mm and .30, register it as a 9mm and bore it appropriately.
As for length:
The vast majority of "solvent trap" kits out there (since you mentioned not being able to do tight tolerances) are set up to be a specific length with both caps installed. You buy a 7" tube and the tube that comes will actually be more like 6.5-6.75"
If you are machining everything yourself, you no doubt have some knowledge of trignometry. No CAD needed to draw out a sketch of your design with accurate dimensions. If no knowledge of trig, most cans are small enough to be drawn 1:1 on letter sized paper. Break out the ruler and protractor and get to work!
I would recommend that, if your can doesn't end up matching what is on the form, you go ahead and submit a letter notifying the ATF of the discrepancy. From another, not-so-high-road forum:
Here's the situation: I filed my Form 1 and my check has been cashed. Now I'm just waiting for it to go pending. I intend to make a mini or micro suppressor, so I put down 5" OAL. After some more research and design I've decided that 5" might not be feasible given that I want this to use a QD mount. I decided it might be best to amend my Form 1 to make it a little longer. So, I called my examiner, Nikki Dudash. When I spoke to her she told me that for Suppressors on a Form 1 "the overall length isn't a big deal." She said that if this was a short barreled rifle or short barreled shotgun then it would be an issue. She informed me that I had a few options since the difference in length is so minor:
1) Have them return the Form 1 to me and I could create a new one and send it back in with the new information on length;
2) Just leave it as is and not worry about it; or
3) Call them after I have been approved and let them know that the overall length of the item has been changed from 5" to 6". She said however, that there is no need for me to send them any notification that the OAL has changed, but I can do so if I would like to.
I wasn't entirely sure this was true, so I went ahead and spoke with Ted Clutter. He's informed me that of the three options I listed above, I would be best advised to draft a letter to the NFA Branch informing them that overall length of my Form 1 suppressor has changed. He said that this is similar to notification that would be provided when the overall length of a short barreled rifle or short barreled shotgun changes. He said that the goal is to keep the registry as accurate as possible. He said approval for the silencer will not change based upon a change in length, much like a change in length of an approved SBR will not change it's legality. I told him I will send him a letter detailing our conversation and requested that he please provide me with confirmation of the contents of my letter.