Your wording is slightly unclear but I'll answer on the basis that planet is 1.5x bigger (1.5x the radius) and 1.5x more massive than the Earth.
Now the acceleration due to gravity F = GM/r^2
Compare your world with Earth: Fworld = G(1.5M)/(1.5r)^2
The surface gravity on your world with be 1/1.5 times that of Earth, i.e. only 67% of Earth gravity or about 6.7ms^-2. This is a low gravity world, not a high gravity one.
That should suit your megafauna and megaflora quite well. Note that the tallest tree on Earth is the giant sequoia which can grow to 115m tall, so your suggested size for biggest trees on your world may well be a big underestimate, you might find the big ones reach around 150m. Terran giraffes reach 5.8m and clearly taller creatures than giraffes existed in the past (tallest ever currently believed to be Sauroposeidon at 18m tall), so 9m for a large creature on your world doesn't seem out of the question.
Now this all means that the planet has a lower density than Earth, probably specifically meaning a predominantly silicate rather than iron core. That may suggest it's a rather mineral poor world, though you could easily arrange for a suitable heavy bombardment soon after planetary formation to ensure the planet had been provided with enough heavy elements in the crust and upper mantle to support a rich biosphere.
Edit: on closer inspection, the maths makes a planet with 1.5 x the radius and 1.5 x the mass extremely unlikely - its density would be about 10.5% that of Earth, i.e. half the density of water. However that's not to say a world with that radius but a similar-to-Earth gravity isn't possible: with a density of 0.67 that of Earth (i.e. 3700kg/m^3) it would have the same surface gravity as Earth. That's a reasonable density for rocks, so you just need to state that your planet is primarily rocky rather than with an iron core, which may in turn mean it has a much weaker magnetosphere and you therefore need to place your planet in orbit around a much quieter star to avoid irradiating the surface.