I have no experience with ebb and flow, but a typical commercial bato bucket with top drip has a volume of about 11 liters or just under 3 gallons, so I would think your 3.5 gallon buckets would work.

I have tomatoes in bato buckets for the first try at real hydroponic gardening and they are growing like weeds using the nutrient that mhpgardener suggests. I am using expanded shale as a grow medium.

I have some in 20L tubs of pea gravel with flood and drain. You can literally just throw tom seeds straight into the gravel and they`ll come up.

3.5 gallon buckets are plenty large for any tomato you can grow in ebb and flow.

Thanks everyone. That is reassuring, as all the soil based advice is to get very large containers.