If a creature evolves an appendage, the advantage this appendage provides needs to be substantial enough to push the evolution of that appendage to completion, eg. a tapir-type nose evolving to an elephant-style trunk as a very useful and multi-purpose breathing and manipulation digit.
A larger body needs energy in larger quantities and roughly more evenly to sustain that size. A proboscis can only suck in liquid substances, necessitating that the creature either feeds exclusively on liquids (like butterflies but you'd, umm, really need humungous amounts of tree sap or nectar to sustain a large-sized body) or liquefies its food using some kind of venom/enzymes (again, needs to liquefy large amounts of food to sustain itself, though not as much as simple-sugar nectar). Any solid material getting jammed in the proboscis is going to be tricky to get out, so the proboscis itself will have to be narrow as a matter of self-protection, restricting the amount that can go in at a time and prolonging feeding time.
If this proboscis is involved in injecting the liquefying venom, it will have to be narrow and sharp to be able to pierce through even soft skin like ours, mosquito-style. If there are insect-like chitin shells, tough hides and scales involved, this appendage will have to be pretty strong to avoid getting damaged.
To get enough nourishment out of a 'kill', it will need to liquify a large amount of mass in a short time. If this means the venom is strong and can liquefy even large amounts of protoplasm fast, the problem of production and storage of this venom will mean proboscis-creature will need specialized organs to deal with large amounts of this highly corrosive substance. Else it means it will have to inject the prey multiple times all over to ensure it liquefies properly. Either way it will need to wait before the prey is sufficiently liquified to eat. This may be a longer wait than it can afford, especially if there are super-predators that prey on our proboscis creature.
The closest to this ambush/trap/attack and liquefy strategy is spider-venom but even a spider eats semi-solid chunks along with partially liquefied matter. The ability to process solid/semi-solid diet via teeth/digestive enzymes enables the creature to ingest more matter, reduce wastage of good edible matter and hastens a creature's departure from the 'kill scene'. There's no reason why a creature should not evolve an injecting apparatus like hollow teeth or fangs but restricting it to feed via this apparatus would turn detrimental to its survival.
So, no, I don't see this creature getting very big, a quarter plate-sized ambush specialist at the most perhaps but then it would have to compete with the comparatively more efficient spiders in that size bracket.