What has worked for you when trying to scale product training to a large field of sales reps at mass?

Pat and Sean did a great job answering with some more tactical approaches so I'll be brief with a couple tips. 

There are purpose built tools like Inkling that can be a great way to enable massive sales teams, but they require a ton of investment to do well. Webinars and quizzes are things that work well remotely. Salespeople are competitive so use that to your advantage.

If you have a massive sales team, you should also have the budget to get some outside help to help train them. I’d recommend hiring a professional services firm to make sure the training doesn’t consume all of your time.

+ 1 to weekly recorded trainings. At Cloudera we did "Sprint trainings" on Mondays for 30-60 minutes and recorded it. It was a valuable way to stay in sync with the salesforce. We also had a marketing newsletter that included a small section on new assets and large one for major announcements + a sales newsletter run by sales ops that included all recordings of sprint trainings.

On top of the weekly training sessions or sprint training ideas mentioned before, I have seen the use of tools like HighSpot has been very helpful. These tools help manage all the sales collaterals and content produced, organize them them using tags and then mapping it back to your CRM at the lead & account level.
This way when the BDR/ISR and the field sales are researching on a prospect and its company in the CRM, relevant documents show up in their CRM view. 

I have seen this being more scalable and having higher adoption among sales than the use of Google spreadsheets. However, before investing in one more tool, will first recommend starting with google spreadsheets to understand the adoption, usage and needs fo the sales enablement process.  

You could even link or integrate these online spreadsheets into your CRM

Do you have a weekly product training with the go-to-market team (marketing, SDRs, sales, and customer success)?

We did this at Oracle and Leadspace and it worked well. It's a weekly, 1 hour live training forum where we introduce product updates, new sales tools, and deep dive into customer use cases. At Oracle, over 350 sales reps joined the weekly meetings and they were recorded, so more people go to see the replay if they couldn't join live. For tech, any web conferencing software would do. But you have to be the one getting input from sales for training topics, setting the agenda, and, finding speakers.

Content will change depending on the needs of sales and where they need the most help. Content could be more messaging focused or product focused.

As a baseline, you should cover this:

1. Targeting* - Who are your buyers, what do they care about, what problems are they trying to solve.
2. Prospecting* - How to penetrate accounts, what's your 30 second elevator pitch, how to book a meeting.
3. Discovery Questions* - What questions to ask prospects to uncover need.
4. Pitch Deck* - How to pitch the product, what to say, how to communicate value in a 7 minute pitch.
5. Product Demo* - See the product, communicate the value of the product with a live demo.
6. Product Features* - Get deep into product features and functionality. What's the underlying technology.
7. Use Cases* - How customers are using the product, what are the results.
8. Competitive Differentiation* - How is the product different than competitors? How do you respond when prospects ask about a specific competitor?
9. Pricing and Packaging* - How to price the product. What packages are available?
10. Roadmap* - What's in the future for the product. The rhyme and reason for this schedule is thinking in terms of a sales process.

A sales rep is going to want to know who to target, what to say, how to differentiate, how to demo, and how to price - usually in that order. You'll cover all your bases, then you can figure out where your team is most "stuck" in the sales process and focus more on that - maybe do a repeat session on that topic.

I find that aside from consistent product training schedule plus the format and certification. Technology will help with the scaling of the training. I highly recommend looking at sales enablement tools, that includ some LMS component, and can help to distribute content as well as TARGET content to sales groups (regions territories). The key is to reference last product training with a recap in any product training. Essentially, your product training should sound like a long narrative, which each product training as a chapter in a book. Recaps can be quick recaps before a training starts, also include a "in case you missed it" newsletter that comes monthly. 

And in regards to certification, I would break down the complexity of the certifcation based on the importance of the product launch. For example, for 0 or 1 level product launch (really important), there is a training, plus sample pitch all livestream and video, and reps have to record themselves pitch said product and submit to some kind of channel or repository. For anything below, I wil usually leverage quizes.