Welcome. Without more context its really a question of how long a piece of string is. Environment, skill level, goal and duration all play a part. For example for me on an average day in the woods where I live, the essentials fit in a Ribz-pack which is about 8L. That is enough to keep me alive long enough to be found. As soon as I'm working with clients, that essentials pack has a volume of about 30-35L. Back home in Canada in winter, it wouldn't be unheard of for just basic gear to take up nearly 70L in insulating layers, and extra calories. On the other end, there are a few chaps here who have the skills such that what they need is either in their shirt pocket, or within view at all times from the natural environment.
If I was to recommend someone a pack size to start with as they learned survival skills without any other context, I'd say 45L should get you enough room for a pretty comprehensive 10-essentials list, as well as some safety margin, while being sell built enough to carry the likely heavy load of a novice, or budget spender. You won't do a week out of one, but a weekend living rough would be very manageable.