Why does it seem like Edward can never create large blocks or large weapons to kill his enemies? I'm only on season 2 so so far, but it seems he always creates spears and arm blades.
Two reasons (one in-universe and one out-of-universe): character definition and familiarity.
Character definition (and easy identification) are part of a good story. Say you see an image from an FMA:B episode and you see flames shooting across the screen but can't see who's attacking. Any guesses as to who's lighting it up? If you didn't guess Roy Mustang, well, we need to educate you in the ways of FMA:B. See pink sparkles and muscles, who is it? Major Armstrong. Red lightning and explosions? Kimblee. The list goes on.
By giving each person a distinct fighting style, it does a few things for the story. It helps people to differentiate characters. It helps people to find someone they can admire / relate to / enjoy watching. It also keeps things interesting. Mustang is an absolute terror to fight against. Until you get him wet, then he becomes the laughing stock of the show.
As far as in-universe reasons, I'm sure it has to do with familiarity. For example, Major Armstrong is always shouting about his "alchemical style that has been passed down the Armstrong line for generations". That's how he was trained to fight. And he's good at it. It plays to his strengths.
But think about it. Could Armstrong create a sword if he needed it? Sure, draw the appropriate circle and poof, sword. But I doubt he would be terribly efficient with it. Look at the way he moves. He moves like a boxer and wrestler. Trying to do that with a sword just wouldn't work. Fighting with swords is different from hand to hand combat. Which is different from using guns or throwing fireballs or ...
One other thing to consider is the need for transmutation circles. Most alchemists keep their favorite one handy. Armstrong's gauntlet things or Mustang's gloves have a circle on them to enable their preferred alchemy. I doubt most people keep a plethora of those handy for whatever they need. That would also imply they have the knowledge to use multiple types of alchemy. While most alchemists can handle the basics, they tend to specialize. There are multiple mentions of alchemists who specialize in things like medicinal alchemy, chimeras, etc.
I know you've asked about Ed specifically and I'll address that now. Ed is a special case. He doesn't need a circle, so he should be able to theoretically do whatever he wants. But, as stated above, he may not know enough about a particular type of alchemy to use it effectively. Just because he wants to use a gun in a fight doesn't make it a good idea. Does he know how to make a good gun? Ammo? Can he hit the broad side of a barn even if he had the gun? No, Ed is going to use what he knows he can be good with.
Not sure if you are there yet, but Ed was trained by Izumi Curtis. And part of that training was hand-to-hand sparring. So Ed is trained in and comfortable with close quarters combat. So his choices in weapons are going to reflect that. So an arm blade or spear are going to suit him nicely. A gun or cannon aren't going to make good weapons. Too much potential for collateral damage, hurting yourself, etc. especially if you aren't proficient with them. Could he try blowing stuff up like Kimblee? I suppose, but he's just as likely to wound himself as his opponent if he can't control it well.
TL/DR: Ed is trained in close quarters combat. So he picks weapons that play to that strength. Anything else would like be a burden or liability if he tried it out in real combat for the first time.
To put it simply: time.
Alchemists, decompose matter, reorganize it, and then recompose the way they want, this requires deep knowledge of the item to transmute/create, in the heat of battle you need something to hit or defend yourself and you usually need it NOW, so, alchemists prefer simple transmutations, that don't require much time.
I can't remember much but there are battles where Ed has some prep time and makes some complex devices.
Mild spoiler Put scar for example, his arm tatoo is a transmutation circle, and he only uses it to decompose matter so he destroys everything, and since he doesn't reorganize nothing, his transmutations are really fast.
source: mostly remembering the manga that I read about a year ago