A simple sentence may have a compound subject or predicate (or both) and may also include a number of modifiers and complements.
Obviously, then, a simple sentence need not be short. It remains simple in structure so long as it contains but one simple or compound subject and one simple or compound predicate. Thus….
1. You leave Glasgow in a steamboat, go down the Clyde fourteen miles, and then come to Dumbarton Castle, a huge rock five or six hundred feet high, not connected with any other high land, and with a fortress at the top. Webster
The length of this sentence is due partly to its compound predicate, partly to the modifier (and modifiers of the modifier) attached to the noun Dumbarton Castle.
2. He was little disposed to exchange his lordly repose for the insecure and agitated life of a conspirator, to be in the power of accomplices, to live in constant dread of warrants and king’s messengers, nay, perhaps, to end his days on a scaffold, or to live on alms in some back street of the Hague. Macaulay
This sentence is lengthened by means of a series of infinitives used as adverbial modifiers of the complement disposed (a participle used as an adjective). Each of these infinitives takes a complement or a modifier (or both).
3. The arbitrary measures of Charles I, the bold schemes of Strafford, and the intolerant bigotry of Laud, precipitated a collision between the opposite principles of government, and divided the whole country into Cavaliers and Roundheads. May
Both the subject and the predicate are compound. Each of the three nouns in the compound subject has modifiers. The two verbs in the compound predicate have each a complement, and the second has an adverbial modifier (a phrase).
4. Twenty of the savages now got on board and proceeded to ramble over every part of the deck and scramble about among the rigging, making themselves much at home and examining every article with great inquisitiveness. Poe
The predicate is compound. The sentence is extended by the use of participles (making and examining), which modify the simple subject twenty.
5. She was tumbled early, by accident or design, into a spacious closet of good old English reading, without much selection or prohibition, and browsed at will upon that fair and wholesome pasturage. Lamb
6. The mermaid was still seen to glide along the waters and mingling her voice with the sighing breeze, was often heard to sing of subterranean wonders, or to chant prophecies of future events. Scott
7. With early dawn, they were under arms, and, without waiting for the movement of the Spaniards, poured into the city and attacked them in their own quarters. Prescott
8. Arming a desperate troop of slaves and gladiators, he overpowered the feeble guard of the domestic tranquility of Rome, received the homage of the Senate, and, assuming the title of Augustus, precariously reigned during a tumult of twenty-eight days. Gibbon
Note : A simple sentence with compound predicate often differs very slightly from a compound sentence. Thus in examples 4–7 the insertion of a single pronoun (they, she) to serve as a subject for the second verb (proceeded, browsed, etc.) will make the sentence compound.