You've hit on a common and frustrating issue. There's no single "right" answer, as the best choice depends on a variety of factors. Here's a breakdown of the pros and cons of each, along with some strategies for dealing with the issues you're encountering

3-0 Outlet Boxes (3" diameter, approximately):
Pros



Traditional & Common

Widely available and often preferred by older electricians due to familiarity.

Perceived Robustness

The slightly larger size may feel more secure to some, especially in older homes.

Coverage Argument (as your old timer stated)

Can sometimes cover slightly larger/sloppier drywall cuts around the box.

Cons



Fixture Fit Issues (as you've seen)

Many modern, smaller, and more decorative light fixtures are designed specifically for 4-0 boxes. Adapting them can be clunky and sometimes unsafe.

Appearance Issues

If the fixture is significantly smaller than the box, it can look odd, even with a well-done installation.

Limited Box Extender Options

As you noted, finding box extenders specifically for 3-0 boxes can be difficult, especially the "cookie-cutter" or aesthetically pleasing types.

Becoming Less Common

While still available, the industry is moving towards 4-0 as the more standard size, meaning availability of 3-0 specific accessories might decline.

4-0 Outlet Boxes (4" diameter, approximately)



Pros



Modern Standard

Increasingly the standard for new construction and renovations.

Fixture Compatibility

Designed to accommodate a wide range of modern light fixtures, especially smaller and more decorative ones.

Box Extender Availability

A vast selection of box extenders and adapter plates are readily available, including flush-mounted options for a clean, finished look.

Better for Modern Wiring Practices

More room inside the box can be helpful when dealing with larger wire gauges or the increased wiring volume associated with modern wiring (e.g., smart switches).

Cons



Coverage Issues (the opposite of your old timer's concern)

If the existing drywall opening is too large, a 4-0 box might not completely cover the hole, requiring drywall repair.

Might not be ideal in older homes

If you are replacing a fixture in an older home and are not opening up the wall to replace the box, you might encounter the 3-0 problem mentioned earlier.

Wisdom and Strategies


Project-Based Decision

The best approach is to decide on a project-by-project basis. Consider the style and size of the lights you plan to use before choosing your boxes.2.

New Construction/Renovation

If you're doing new construction or a major renovation, strongly consider using 4-0 boxes throughout. This will give you the greatest flexibility in fixture selection down the line.3.

Existing Installations



Assess the Existing Condition

Before replacing a fixture, carefully examine the existing box and the drywall opening around it.

Fixture Research

Once you've chosen a fixture, check its dimensions and mounting requirements before buying anything. Many manufacturer websites have specifications and installation instructions available online.

Adapters and Extenders

Explore your options for adapters and extenders.4.

Adapting 3-0 to 4-0 (when using 4-0 fixtures)



Adaptor Plates

Adaptor plates are an easy way to bridge a 3-0 box to a 4-0 fixture. Simply mount the plate to the 3-0 box and mount the fixture on the adaptor plate.

"Mud Rings" (Plaster Rings)

Although the name implies plaster, mud rings can be added to a box to allow you to use a variety of light fixture sizes.5.

Adapting 4-0 to 3-0 (for covering large holes with 3-0 fixtures)



Drywall Repair

The best and most professional solution is to properly repair the drywall around the existing opening. Use drywall patches or shims to create a solid, level surface that the 3-0 box will cover. This is the most time-consuming but yields the best result.

Oversized Canopy Fixtures

Look for light fixtures with canopies (the part that covers the box) specifically designed to be larger and cover imperfections in the drywall.

Custom Adapter Rings/Plates

For the sake of appearances and safety I would not recommend this. But, it is possible to have someone custom make a ring or plate from metal or even 3D-print a ring or plate. If you decide to go this route, be certain to use materials that won't degrade and/or become flammable over time.6.

Consider Your Clientele (if applicable)

If you're working for customers, consider their preferences and aesthetic goals. Show them both box options and explain the trade-offs.7.

Safety First

Always prioritize safety. Turn off the power at the breaker before working with electrical wiring. Ensure all connections are secure and properly grounded. Don't overload the box with too many wires.* If you are uncomfortable with any aspect of the electrical work, hire a qualified electrician.

In summary:

While your old timer's experience is valid, the industry has evolved. 4-0 boxes offer better compatibility with modern fixtures and are increasingly the standard. However, understanding the pros and cons of both and having strategies for adapting them to different situations is key to a professional and aesthetically pleasing result. Choose the appropriate box based on the specific requirements of each project, and don't be afraid to use adapters and extenders when necessary. Flag for review