Right now a maximum block size is nowhere near enough to accommodate all of the transactions trying to get confirmed (legitimate transactions or otherwise), you can look at the size of recent blocks to see that recently found blocks are all 950kb+, and you can look at the mempool of any reputable block explorer to see that there are several MB worth of unconfirmed transactions and several BTC worth of transaction fees within these unconfirmed transactions.

If for example, we were to increase the maximum block size to say 10MB, (which I believe to be safe currently), then blocks on average would probably be about 10-11% full or so. If transaction growth were to grow at say 30% per year (which would be huge, and probably higher then what is realistic), then after about 8 years then the maximum block size would need to be increased again when the demand for transactions would create the need for blocks to be about 90% full.

In ~8 years, we could raise the maximum block size to say 50MB to accommodate for future growth of transaction volume. Now with today's network technology, with today's cost of storage, and with today's cost of processing power, 50MB is probably be too large, however over the next 8 years, we will almost certainly see increases in all of the above, and the cost of all of the above is almost certain to decline. Since transaction volume has already grown to be very large, after 8 years, you might want to assume that transaction growth slows to 20% per year (still very large). After ~9 years of growth, the maximum block size will again need to be increased, and again the technology regarding networks, storage and processing will almost certainly have improved and the cost of the above will almost certainly have declined, allowing for an increase of the maximum block size without increasing the cost of running a full node from the time the maximum block size was last increased.