Starting this year I have been front loading two panniers on my commuter, with a reasonably large load (laptop, papers and project books, clothing (work and change) and food). For what I have learned is that there isn't a single answer to how large you can go and depends on a number of factors:
Q1: Pannier Size
I actually run rear panniers (and/or a computer pannier similar to your linked one) on the front (Orlieb rear panniers), and have been through three different low-rider front rack set-ups until I found one that worked with these larger sized panniers. The main problem for me was the smaller sized front racks didn't properly support a larger rear pannier (see Other Considerations Item 3).
This was a difficult process as this is a somewhat non-standard setup the on-line information information was inadequate at best. For example, I started with a Tubus Tara, which does not properly support larger panniers (as suggested by one of your LBS), but the Tubus Duo (you need fork that has inside and outside eyelets) has the lower rail positioned about an inch lower than the Tara which does support larger panniers. NO WHERE was this mentioned in any of the Tubus literature. I figured it out by going to a local bike shop and holding up different racks to my bike and my panniers. No one in the various bike shops had any idea about these differences, as the standard touring setup is to run smaller panniers on the front, with a larger load in the back.
I also tried top platform racks like the Surly Nice rack, which did a good job of supporting larger panniers, but I didn't like how much it flexed when trying to steer under heavier loads (this makes balance difficult). The higher pannier position compared to a lower rider made the bike feel less planted.
In the end answer seems to be a lot of trial and error, with little help out there because it is a non-standard setup. You will need to ensure your rack can properly support your pannier size and that you can properly fix the pannier to your rack (see Other Considerations Item 3). If one set-up doesn't work, don't give up hope you may want to try a different rack or pannier combo.
Q2: Max Size
By size I assume you are talking about weight (as your first question asks about size). As suggested by other answers it really depends on if the bike still handles well (see Other Considerations Item 1).
While you didn't explicitly ask for general advice, I feel it is worth adding the following general notes about front-loading only as it is a rather niche approach:
- Steering is affected by front loading, it works best with a bike designed to front load. There seems to be two approaches here, (a) a low trail geometry (e.g., Soma GR) and (b) shifting the riders weight farther back by default so a front load balances the weight distribution (e.g., Specialized AWOL). If your bike is not designed to be primarily front loaded (e.g., Soma Saga) front loading will make the bike sluggish and hard to steer, this gets worse as the load increases.
- You will need to ensure your panniers are evenly loaded or your steering will pull towards the heavier pannier. Carefully loading is less of an issue when touring, just part of the preparation like packing down your tent, but to can be a hassle when commuting. When rear loading the panniers, uneven weighting has a much smaller effect on handling.
- Fixing the pannier properly to the rack is also critical in a front loading set-up. If you only partially fix the pannier (or the pannier is not properly supported) it can twist into the spokes. If it jams, it will result in a rather nasty crash. The same scenario in a rear-loading configuration is much less dangerous as in the worst case it will simply result in a skid and a damaged wheel or pannier. When commuting and tired in the morning it is easy to make mistakes such as not properly fixing the pannier.
- So why even front-load? If the bike has a front loading geometry the ride quality is much better than rear loading. Climbing out of the saddle feels very natural and similar to a bike without a load. Plus it is easy to skid/slide the rear through corners. Killer style points!