If you makes everything in universe a size bigger..

[With a box]

Would it make significant difference?
Would things be more durable or more fragile?
in this buff to melee?

[Tommy2255]

5 foot step is still 5 feet, so most creatures will not be able to get away from a melee opponent without provoking an AoO. Definitely a significant buff to trip builds. However, speed usually doesn't increase as fast as size does, so it would be a debuff to chargers since everything is now further apart.

[Eloel]

Would it make significant difference?
Would things be more durable or more fragile?
in this buff to melee?

Lower levels would get a lot deadlier, with damage scaling way faster than HP at level 1. Even assuming a simple +2 to strength from size a greatsword would deal 4-5 more damage on average, which, even if it's a single hit, means you need a lot of Con bonus to counterbalance.

[Mr Adventurer]

Medium to Large means +8 Str, -2 Dex, +4 Con, and +2 NA IIRC

[Necroticplague]

Would it make significant difference?
Would things be more durable or more fragile?
in this buff to melee?

Yes.
More fragile.
Depends.

[Chronos]

Stealth will become a lot harder, since Hide is penalized for size, but Spot isn't.

[Shoat]

Would it make significant difference?
Would things be more durable or more fragile?
in this buff to melee?

Generally, larger things are more fragile and weaker (their strength scales slower than their weight, their nutrition-requirement scales faster than their ability to gather food, etc.).
Most lifeforms would not be able to remain alive if scaled up one d&d size category (heart too weak to make blood reach everywhere, lungs to weak to supply body), but even if we ignore that and concentrate only on game rules everything will get more fragile and weaker (damage goes up, durability doesn't).