humidity will be a problem, if at all, when the plants are full size and transpiring at faster rates. the moisture gassing off your fabric pots will be a small factor and not much different than a plastic nursery pot. the benefit with the fabric is teh air-pruning. it's very difficult to get rootbound.
depending on your local weather and such, you may or may not need a dehumidifier. The size of your garden will be a major factor as well as the size of the room and whether you exhaust outside or not.
you definitely want a dehumidfier that's larger than required. You don't want one that struggles to keep up. It'll just die faster. if underpowered you'll likely have mold issues.
if you have a 4x4, probably a 30pt (2019 rating in usa - "AHAM") is probably fine. Beware, AHAM is 60F/60RH rating... if they give a rating at "saturation" it'll be a little more than 2x larger pints/day rating. You want to compare AHAM rating, not pints per day at saturation.. that's just a useless exaggeration "at saturation".
So if you see a big price difference but the pints/day rating don't jive, it's probably because one is giving AHAM rating and the other is giving saturation rating. If they completely leave this information out, they are a shady AF brand to be avoided.