Yes they will be shorter (VIA currently uses 5-7 car trains along these routes). The question is will they be short enough for DMUs to make sense? VIA will have the best information for determining this, but looking at
VIA's Total passengers at stations (boarding and deboarding) in 2018 for the top 6 stations along the "lakeshore" corridor (ignoring Toronto, Montreal, Ottawa, Fallowfield and Dorval as they will have HFR, but including Oshawa as it won't).
Rank | Station | Total Passengers |
5 | KINGSTON | 456,586 |
9 | OSHAWA | 207,037 |
12 | BELLEVILLE | 146,395 |
13 | COBOURG | 136,541 |
18 | BROCKVILLE | 61,305 |
20 | CORNWALL | 55,890 |
| Total | 1,063,754 |
Now if you divide that by 52, you get 20,457 passengers per week. Assuming ridership remains the same, and that there will be 12 trains a day weekdays, 10 trains on Saturdays and 8 trains on Sundays (a guesstimate) each way, that is a total of 78 train each way or 156 trains total per week. Divide that out and you get an average of 131 passengers per train, using 2018 ridership. With a schedule that tailored for the lakeshore service rather than to/from Ottawa and Montreal, that number should grow significantly (this is just a starting point). One of VIA's new "Extra short," 3 car trains will have a capacity of 176 passengers, so that will be a good starting point, and more cars can be added seasonally as ridership grows.
I admit these calculations assume that no one is traveling between those stations (instead they are going to/from Toronto, Montreal, Ottawa or one of the other stations not listed), but I also ignored the smaller stations (like PORT HOPE, GUILDWOOD, TRENTON JCT, etc.), so that should balance out reasonably well.