The regions of the InputBoxTemplate template are "fixed" to a certain height. You could try using a backdrop instead.
Code:
local editBoxSpells = CreateFrame("EditBox", "DoTrackerEditBoxSpells", menu)
editBoxSpells:SetBackdrop({
bgFile = [[Interface\Buttons\WHITE8x8]],
edgeFile = [[Interface\Tooltips\UI-Tooltip-Border]],
edgeSize = 14,
insets = {left = 3, right = 3, top = 3, bottom = 3},
})
editBoxSpells:SetBackdropColor(0, 0, 0)
editBoxSpells:SetBackdropBorderColor(0.3, 0.3, 0.3)
editBoxSpells:SetMultiLine(true)
editBoxSpells:SetSize(300, 300)
editBoxSpells:SetPoint("TOPLEFT", 300, -107)
editBoxSpells:SetAutoFocus(false)
editBoxSpells:SetText("test")
editBoxSpells:SetCursorPosition(0)
editBoxSpells:SetFont("Fonts\\FRIZQT__.TTF", 10)
editBoxSpells:SetJustifyH("LEFT")
editBoxSpells:SetJustifyV("CENTER")