public class StringChunkingWithWordDetection
    {
        /// <summary>
        /// This also detects words by using a list of space indexes
        /// And reduces the chunk size appropriately
        /// Probably should have used a Natural Language processing lib but already done now
        /// </summary>
        public IEnumerable<string> Process(string str, int maxChunkSize)
        {
            var wordDetectedString = new List<string>();
            foreach (var word in str.Split(' '))
            {
                if (!string.IsNullOrEmpty(word))
                    wordDetectedString.Add(word);
            }

            return ChunksUpto(wordDetectedString, maxChunkSize);
        }

        private IEnumerable<string> ChunksUpto(List<string> words, int maxChunkSize)
        {
            var lastWordIs = words.LastOrDefault();
            var chunk = "";

            foreach (var word in words)
            {
                var lastWord = lastWordIs != null && lastWordIs == word;
                var separator = lastWord ? "" : " ";

                var previousChunk = chunk;
                chunk += word + separator;

                if (word.Length > maxChunkSize)
                {
                    chunk = word.Substring(0, maxChunkSize);
                    yield return chunk;
                    chunk = word.Substring(maxChunkSize) + separator;
                    previousChunk = "";
                }

                if (chunk.Length == maxChunkSize)
                {
                    yield return chunk;
                    chunk = "";
                }
                else if (chunk.Length > maxChunkSize)
                {
                    chunk = word + separator;
                    yield return previousChunk;
                }
            }

            yield return chunk;
        }
    }