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What difficulty and size should i go for playthrough
Sounds like you've decided on Expert Mode which seems like a good challenge if you're new to the pure summoner class.
I personally find small worlds to be too small even on SP. World gen also can make some weird natural structures on small worlds. Would recommend Medium-size, but it would probably work if you did small or medium for a Single Player playthrough.
I will say Summoner is ASKING for hell in pre-hardmode. Mostly not enough DPS, but by hardmode. It is about thinking of stratedgies. The summoner was nerfed slightly in 1.4 and buffed also. Nerfed as all summon weapons ONLY TAG if holding summoner whips or staffs. Though good luck if get the vampire frog, that worthless summon needs to GTFO.