You can make the MQ MZT arrows a bit larger in NinjaTrader 8. If you open the Data Series and increase the Bar width input, you'll see a corresponding increase in the symbols, including the arrows. This works because NT arrows are drawn relative to the bar width. Note that this might be tempered by bar spacing settings within the Indicator settings, but you can experiment with what works best. There's a limit on how wide you can make the bars as you'll want to avoid having the bars overlap.
Note that you can also zoom in on your NT charts to increase the size of most of the chart components. These techniques will likely work for NT7 as well.