This is a bit of a weird question.
In North America we have two spiders that have medically significant venom - the Black Widow and the Brown Recluse. They are both small spiders, but rarely lethal. In fact, for most health youth and adults a bit from a Black Widow is only incredibly painful. The mechanism by which it causes death is actually because it causes paralysis, typically the diaphragm, causing suffocation.
I'm not aware of a brown recluse bite ever being responsible for a death, and they are also the most misdiagnosed spider bite.
All of that is background information - either your killer pholcid spider (not to be confused with a harvestman, which is an arachnid but not a spider) is going to require a different venom, or it's going to need to go through some pretty radical changes.
We know that their venom is not particularly potent. Unless you have some sort of severe allergy, you might get a mild burning and some redness in the extremely unlikely case of actually getting bitten.
So if we rule out any changes to venom (aside from perhaps the amount produced, due to size changes), we've basically just got to go with sheer size.
If you've hung around worldbuilding for long you'll know that the square cube law comes into play a lot when we're scaling stuff up. I don't know if the numbers could even pan out, but basically what we're looking for is going to be fang size. According to that Wikipedia article, their fangs are 0.25mm in length. Let's say we need their fangs to be 100 (about 4") long, based on the typical legal knife-length limit.
That's 400x the size. That means that their 10mm body will be 4,000 - or about 160 inches, or 13 feet. Their legs will be 20,000mm long, or 65 feet. That's just ridiculous. At that point they're probably able to cut you with the claws on their feet more effectively than using their fangs.
This says nothing else about the physiological changes they're going to need - their book lungs will no longer be effective, their hydraulic blood system will be problematic... in general, they're going to have a lot of problems, even if you can handwave that way.
Although on the plus side, they will definitely be large enough to kill an average human.