...acting like it had an almost dead battery with only just enough energy to turn the interior lights on. I took the battery to O'reilly auto parts to have it charged up which they did but also told me that it isn't the right size for my T4R although it's been seeming to work fine for several months now. ...it's back to the same non starting way it was before except now i can turn the alarm on and off. Anyone got any ideas as to what to do?
By non-starting, you mean? No crank? No clicks? No spark? No fuel?
Short answer - take the alarm out.
Long answer - don't get ahead of yourself. Get the correct size battery for starters (get it? hah!). Then go digging through your alarm to see if it's immobilizing your ignition. Then determine if the battery is being drained by a short in your wiring, then check that you're getting juice to the starter, etc. Take it step by step, starting with the most logical/easiest to test causes.
Agree with KidV.
Trust the OReilly associate and your suspicion, but verify. How many cold cranking amps were on original battery? Does existing battery satisfy that? If you have no docs, you can start by saying what truck year-model you have so other members can tell you what CCA your battery should have.
What was the last thing you did to truck before this problem occurred?
Intermittent no start with alarm could easily be caused by low battery voltage.
If battery is undersized, you can usually tell because when starter starts to crank, if would "groan", struggling to turn an engine too heavy for it to turn, and lights would definitely dim. OR starter solenoid (piggy-backed on starter motor) would click, and lights would dim- meaning solenoid contact completes circuit to starter but there's not enough power to even move the starter.
Try load-testing battery yourself.
With all switches off what's your battery voltage?
Turn on headlights to high beam. What voltage did your battery drop to?
Turn Ignition on, headlights high beam and AC on. What voltage do you get?
Compare readings to those of members with same model-year and known healthy batteries.
Good luck.
P.S.
I have a simple first-gen so
screw the alarm. No sense in buying a truck for its simplicity only to complicate and make it less reliable with more electronics. I simply put kill switches for 3 systems. The more questionable my parking area, the more kill switches are off

OR when I feel like it, I just kill all 3 systems (actuating switches often removes oxidation from contacts)
=D