What heat mat size would be better?

i have a 100x30x35 glass terrarium and the 2 sizes i have to choose from are 28x28 or 28x45. Now i know that its supposed to cover 1/3 of the terrarium but looking from the current one i have which is a 28x15 the 28 is more like 25 in the heated area so its gonna be the same in those ones i get.

Btw i hear people talk about heat gradient, in my case for some reason its 30 degrees where the heat mat is and 5cm further away it goes down to room temperature is this normal? there is no gradient basically only a hot spot and the rest is cold

All dimensions are in centimeters and Celsius.

"A Hot Spot", and room temp is typical.

Make sure there is at least 1 hiding place directly over the hot spot and the temp reading of 30 should be the actual temp inside that hiding place. Make sure there is also a hiding place somewhere on the cooler side, and the temp inside that cooler side should not go below 22. If your room temp gets much cooler than that, it's ok to use 2 heat pads. That's actually what I have to do, as it gets very cold here. I have 1 heat pad on the warm side, and another on the cooler side. I keep my temps at 29.5 and 23.

ye he does have 5 hides and a log he can hide into and i know the temperatures i did my research. My question was if there is a downside to the bigger heat pad like when the weather gets warmer it might warm the cold side as well or something like that

guys one thing i learned heat mats are dangerous you must use a thermostat on the heat pad it came with the kit zoo med a rheostat won't cut the power enough to go below 93 but with a thermostat will work great.

i been using heat lamps 1 blue and 1 red both 60 watts on a 30x12x12 glass tank i got 3 hides,i was going to install my heat mat with the zilla thermostat to range between 80 to 87 degrees for extra heat,but i went and bought a infrared thermometer and i found out i don't need a heat mat my temps are awesome on the cold end is 76 f inside the hide is 75 then towards the middle 77 to 80 and the hot spot on the aspen is 82 to 85 but on top of the 2 hides that my snake Daisy likes to bask 1 is 89 f the other is 85 f Daisy seems to love it.im lucky that i have a small kids room empty so with my central heat /air i can keep a good ambient temp when im at work in the evening if the bulb ever burns out the snake Daisy has enough heat while i come back from work as i keep that room 75 to 78 f.

in my opinion snakes like to bask in the wild you always see them on top of tress,on the ground or on a rock right underneath where the sun is shining Daisy is not diferent m always at work in the afternoons so i dont see my snake doing anything but sleeping on the cold side but today i saw her basking late afternoon 3pm she basked for about 2 hours then right now shes on top of the hide in the cold side lol i would recommend lights unless the tank is in a very cold room then you would need a heat mat with a thermostat.