Currently, the only way to fix the invisible, extremely large, extremely small, or otherwise disproportionate objects is for an experienced 3D modeller to take the models, remake them, and port them back into the game with fixed proportions. Until we get another modeller to do this, your best hope is to rescale the objects manually. In-game, the objects now have a scaling option accessed by a button in the lower-left or by clicking R. Then, it will show some new sliders, similar to the Rotation and Transform sliders. For "invisible objects", first zoom out and make sure it isn't extremely large and your camera is clipping inside of it. If not, go back to the white cube for the scaling slider and continue to resize until an object appears. Your object will no longer be invisible.

It's hard to tell when an object will be the right size for a player to fit through, so put a player spawn next to the entrance of the building and see if a player can fit. If they can, great, you put the object to the correct size. If not, continue to resize until you reach the desired dimensions.

For disproportionate objects that seem way too tall, too wide, or maybe too short or too skinny, use the sliders on the sides that are colored red, green, or blue. It's hard to tell when it is at the correct dimensions, so eyeballing it is required.

If you desire more assistance, I will *attempt* to do whatever I can and answer questions on my profile page. I am a map-maker myself and I really hope I can finish a huge Fallout-themed map I'm making using most of these objects.

Thanks for listening,
Rogue